When I was messing around with different sauces, I too had heard about the "bitterness" associated with seeds in the sauce. So, I used to use a food mill to extract the seeds but, in the end, I didn't notice any bitterness when I got lazy and just left them in. Eventually, laziness won out. However, having said that, I wasn't thinking the seeds/jelly had anything to do with more sweetness and just attributed it to the current batch of tomatoes that I've found. For reference, I now use the Cento Italian tomatoes "From Italy" ("ROA" inkjetted on the top of the can).
Some spices, a quick blend in the food processor and a brief heat-up/cool down just to release the spice flavors and my sauce is good to go. Yes, I've tried cold sauce but I think my pizza cooks too quickly for the (dried) spice flavors to release in the oven (typical, a one topping pizza is in my 550+ degree oven for about 6 minutes, with one turn at 3 minutes to provide even cooking). I've never done a pre/post weigh but I also believe the brief heat-up removes some of the dreaded water content from the sauce also...