chopped tomatoes - seeded or not?

I agree with the other posters that it depends on what you are making. For example, when I make pico de gallo, I remove the seeds/jelly, and the skin as well -- I use the "meat" of the tomato, so that I don't have excessive juice/liquid in the finished product.

I think it depends on the dish and your tastes. Sometimes it also comes down to a matter of how many seeds you are dealing with in the tomato. Roma or Italian style tomatoes don't have many seeds to start with, but some slicing varieties are loaded with them. The flavor is in the gel or goo, so tossing that out leaves you without much tomato taste. Cooking also seems to mellow the potential bitterness of the seeds, so in the case of paella or pasta sauce I usually don't bother. On the other hand, if I make a Turkish shepherd's salad, Greek salad, or some other sort which contains a lot of them- I will. In that case, I collect the removed seeds and goo into a strainer, and rub it through with a rubber spatula into a measuring cup or bowl to collect the juices while leaving the sometimes bitter seeds behind. The liquid gets used to make the dressing, so no flavor gets lost. If I get more juice than I want for use, I save it for later use. You can freeze it in a small container or ice cube tray- my favorite method- and then use it in place of water for dressings, cooking grains, etc.

Now that you mention that, haven't seen a recipe that calls for seeding a tomato in some time, don't think it's done often anymore (unless you're using a food mill or some such)

It depends upon your recipe and your individual taste. My preference is to chop with the seeds and jelly. The seeds add fiber to your diet. The jelly adds flavor and moisture to your dish. If you don't want to add them, consider chopping, then straining out whatever juice develops and add the strainer contents to your dish, reserving the liquid. You may need that juice at the end and it is more flavorful than adding in water. Reading through this recipe for paella, I would not seed, just chop the one tomato and add to the dish.