3756 used #25 chain last year without issue. It does stretch quite a bit, meaning you have to either remove links or have some sort of other tensioning system to adjust every once in a while.
I personally love working with #25 chain... it feels almost like working with jewelry compared to #35. As mentioned by Joe though, it's quite a bit more picky in the way of sprocket alignment. We used 1/6" plastic spacers (from McMaster IIRC) to line them up, and they never popped except for when one of our sprockets came lose.
We use the #35 chain and sprockets that we have sitting around from the KoP for prototyping and practice bots, but I don't see us ever leaving #25 and 1/16" spacers for competition. (Edit: We had undersized aluminum spacers, leaving space for adjustments with the small plastic ones).
Here's a picture of our 2012 drive train. We had slots in the wheel brackets for tensioning. Worked well, but a pain in the rear to adjust in a 10 wheel drive -.-
