So first tri coming up in Sept (slightly sub-sprint distance), and on the registration app, of course they ask your swim time for XX distance. I just responded "slow" (which is completely true) and will probably be seeded dead last.

But, until I am really competitive, so maybe a year or more of doing this, does my seed position actually matter? A tri is just a big long multi-sport time trial anyway, so what are the benefits of being seeded any higher than last (under the "not an incredible swimmer, otherwise strong all-around athlete" condition)? I figure if I start last I have lots of motivation to bike/run faster!

The serious part of this is will this seeding matter for my level of competition now?

actually depending on the race you could start or last. When Registrations ask what you think your SWIM time will be, this probably means they will be going one at a time or setting up some kind of a wave system. (is the swim in a pool? then they put the fastest first and try to not have any major buildups. if it is bigger than a pool then I don't know why they are asking) On the other hand, if the race directors ask for an overall estimate, then they might put the fastest last, just like in a cycling TT... although I have never heard of any race that does that.

oh and to answer your question: No.

It won't matter, they probably just want you to be in a group with swimmers you are equal with. That way you won't hold up faster swimmers behind you, and you won't get kicked in the face by slower swimmers in front of you.

But, you are right. It's a time trial, your time starts when you do.

This. Just wait until you do a HIM and are grouped in large waves only to be taken over by a fast wave from behind.