I have topped my tomatoes in the past, and it was around this time of year. However, on the "growing tomatoes" forum, here on the Garden Web, they don't recommend it.
My tomato cages are 5ft tall and I used to trim at about 18" up from that. This year, I'm going to let some "collapse" down the outside of the cage and see what happens. Some may get a little pruning.

The guru's on the tomato forum explained that you don't gain that much my trimming them back. Seems odd to me, but they are more intense tomato gardeners than I am. There are several threads on this topic on that forum. Check them out.

Below are some of my tomatoes. Most of the plants in the foreground are a little over 6ft. This picture was taken about 2 weeks ago. Yesterday I noticed that some vines have collapsed. If they tear open, I'll prune them off.

Thanks for responding. I actually did post the question on the Tomato forum after a few days of no responses here.

I didnt have anyone say not to do it, and several people who do start pruning in August.

I decided to go for it. It makes sense to me. If flower to ripe time is in the 60 day range then anything that hasnt started by now will not be likely to be edible before frosts kill the plants.

I know from experience with Dahlias, that if you do some dis-budding, you get larger, more vigorous blooms from the ones that are left, so I dont see why that wouldnt be true for tomatoes too.

I only have part sun to grow my plants in, so they get extra tall. They are mostly in the 8ft range now. If I let them keep growing they will be 10-12 feet and much more susceptible to breaking. I lost several branches full of tomatoes last year in fall thunderstorms.

I check my plants daily (only 8 of them) so its not that hard to pinch off new growth, and if the flowers that exist now all (or mostly) form fruit, then I will have 40-60 tomatoes per plant which is more than enough for me.

By the way - your plants look great!

Tomatoes do not sun-ripen, they vine-ripen. As such it is better to have as much foliage as possible shading the fruit. Topping reduces foliage, I see nothing gained by doing it. Usually around Labor Day I will start pinching off new flower clusters and clusters of tiny fruit for the reasons you mention, but I am not religious about it. Have not seen much benefit in doing so, doesn't bother me to let the plants simply do their thing until frost and short day lengths do them in.

Hmmmm..........Well I have seen lots of pics of your gardens and know you are a great grower. The big difference in my garden is the amount of sun I get. Sun scald is simply not an issue in my yard, even in our hottest sunniest summers, and this sure isnt one of those.

I know that the leaves via photosynthesis are what produces the sugars for the plant, but I am assuming that the plant knows the right ratio of leaf /fruit it needs as it grows and additional foliage isnt really needed to feed the existing fruit.

I guess I am committed to trying this for the season. If I had dozens of plants growing in full sun, I wouldn't bother, but with only eight it isnt much work to baby them.

"...the plant knows the right ratio of leaf /fruit it needs as it grows and additional foliage isn't really needed to feed the existing fruit."

Great point, but we should maybe take into consideration that young healthy leaves are continually being produced to replace the old worn out, damaged, or dried up leaves at the bottom of the plants. It makes sense to me that young healthy leaves are much better able to do their processes but I do not know that for a fact.

My brother does not have a garden but he always grows three tomato plants every year. He babies them, and always tops them mid summer (mid July) and swears by it. He gets nice tomatoes but always complains about how few he gets, and doesn't agree with me that there might be a connection. ;)

It is a good experiment, I do not see any harm in trying it for yourself. Please post the results and your opinions at the end of the season, I am sure many folks would like to hear about it. :)

-Tom

If you only have eight plants why not top some of them and leave some to compare? And let us know the results.

I have not topped my plants as they get less sun every year due to a growing large tree. This year I planted four in another area and they are much larger with more tomatoes.

I have brought in green tomatoes to ripen in the fall. They continue to ripen into November and while they are not as good as vine ripened tomatoes they are still better than store boughten ones.

Tom - what you say about the new leaves possibly doing a better job makes sense, and if I was trimming off lots of old leaves from the bottom I might re-think allowing some new ones. However this year, for the first time, I have hardly had to remove any bottom leaves from my plants. They continue to look amazingly healthy.

I already have more tomatoes forming than I did last year, its why I am comfortable with not letting anymore begin to develop

mnwsgal - All eight of my plants are different types, so I wouldn't get any real info by topping some and not others this year.

Lack of sun is one of the reasons I do want to keep the plants pruned. In the past they have become so tall and gangly that I can't keep them staked well and branches brake off during the fall thunder storms. A couple are close to 8 ft now, Last year they turned into a big mess by the end of the season. I have been selectively pruning all along this year to keep them more open for better air circulation and make it easier to keep them supported.

The only thing I will be able to report back on this year is whether I get all of my tomatoes to ripen instead of having 100 green ones when frost hits.

I'll let you know.

Anyone else having the problem of ginormous plants this year? I can hardly pick mine and the 5ft plus tall cages are falling over even though well staked.
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Here you can see 1/3 of my plants compared to tall sweet corn. The tallest tomatoes at the end are Sungold.