I don't taste it but some say that skins and seeds cause bitterness. I use my mid season tomatoes for sauce because flavor drops off at the end of the season here.

What varieties have you been growing?

I like acidity (that lovely tomato zing), but not bitterness. When you cook down a sauce, that bitterness is definitely from the seeds; variety makes no difference. As I chop my toms for canning, I squeeze out the seeds and pulp, then run the glop through my mill (you could use a sieve if you don't have a mill), put the juices back into the pot and toss the seeds. The whole tomato goes through the mill for sauces to remove skin and seeds. You'll see a marvelous improvement in the taste of your sauces if you remove most of the seeds first!

I'm with the others re seed cause bitterness. The best investment you can make is to purchase a juice/sauce machine. Here is one example. You can find similar machines from many sources. This one is cheap but effective for home canning.

juicer from Johnnys

DarJones