It's up to you. Tomatoes pollinate/fertilize before the flowers even open up, so the odds of an insect-induced cross are fairly low since the flowers usually are fertilized before an insect even can reach them. If you don't grow in isolation or with bagged blossoms, you will get roughly 2-5% cross-pollinated (by insect) seed from an average plant, but some people save seed forever without bagging or isolating and rarely experience any trouble with the plants produced from saved seed. Some of the older (as in very, very old) open pollinated varieties that have exerted stigmas probably have a higher chance of producing crossed-seed. As long as the seed is for your own use and you aren't distributing it to anyone else, there's nothing wrong with saving seed without isolating/bagging, but there always will be that slim risk of having plants taking up space and not producing something you want.

I would not use saved seeds for trades if they weren't grow in isolation or bagged because it isn't fair to the recipient to give them seed that might not be true to type. However, some great varieties like Earl's Faux, Little Lucky and Lucky Cross all have come from accidental, likely insect-induced, crossing and they are great varieties---so you just never know. Sometimes happy accidents like those varieties become big favorites of heirloom tomato growers once they are selected and stabilized.

I believe Dawn's figure for the percentage of crossing is probably right, at least for this part of the country. Back in 1983 I grew something like 20 varieties, close together and without protection. I saved seed from them all and replanted in 1984. Of about 120 plants I had perhaps 2 obvious crosses. Unfortunately, the crosses were inferior to their parents (in my estimation).

I give my tomatoes about 15-20' isolation from other varieties, more for cherry varieties. And, I plant in blocks, choosing my seed from plants which are not closest to the neighboring variety. This has worked very well for me.

I have grown out seed from unprotected tomatoes, given to me in a swap. It is generally good enough to select what's true to type and move on from there. A cross, though, can be tricky to eliminate. I once had a cross of a pink beefsteak to Tuxhorn's Yellow and Red, which is like Hawaiian Pineapple. It took me three or four seasons to rogue that cross out.

In conclusion. I'd say that if you're not too stuck on having pure varieties, it won't hurt to grow your seed and, then, you might even select for pure seed, planting with some isolation, in the future. If you're not that picky about purity and aren't planning to save seed, then go for it. But if a variety is really important to you, and you can get pure seed, then order it and cut the Gordian knot.

Thank you. I am saving my porter and black krim. My yellow pear was separate so I will save those although I accidentally ordered some more. There are a few I am going to grow away from the main crop this year. If I separate by several hundred feet is that good or is another method recommended

Distance isolation for true purity would be probably 1/4 mile, which isn't practical or possible for home gardeners unless they live on a farm or ranch. This sounds like a large distance, but it isn't, especially in wild-open areas with little to no windbreaks and regular, strong wind.

You can grow them just like George does, in blocks 10' apart (and if you have adequate space, then 20-25' apart probably would be even better) with barrier crops in between to distract the pollinators and you'll get pretty pure seed. Common barrier crops include sunflowers, corn, any flowering vine on trellises or TPs, including beans, peas, cucumbers, etc. You can use almost any flower that is highly attractive to bees (bees love blue flowers like larkspur and borage), and squash works well as a barrier when in bloom because bees flock to those large flowers.

To truly ensure purity, just bag your blooms before they open. You can use any lightweight cloth bag that allows free air movement. I use the little bags sold at craft stores or on the bridal supply aisle at Wal-Mart that are meant to hold party favors, rice, etc. You tie the bag over the bloom before it opens. Later, after pollination and fertilization have occurred, remove the bag and mark the stem of the formerly bagged blossom with brightly colored yard. At harvest, use a non-toxic marker (I use Sharpies) to write the name of the variety on the specific fruit that was bagged and from which you intend to harvest seed. That way, once you're back indoors with a bucket full of tomatoes, it is easy to find the ones meant for seed-saving and separate them from the general harvest. Bagging is not complicated, doesn't take much time and works well. You only have to bag the specific fruit from which you intend to save seeds.

Oh good that will be better than putting a few tomatoes in area I rarely go. Back corner.

I don't mind having mixed seed for my family and friends but for sale I want to stick to tried and true.