Timing belts are good, but in my experience they are a little touchy to use. They're great due to weight and size (aluminum pulley=yay), but for rapid direction changed and high tourque applications, aka a FIRST bot driving offensively around a field, they ahve a tendency to slip and chatter. When driving with a skid steer system, the rapid, high speed direction changes have a tendency to make the belt jump a couple teeth, which will wear them down quite quickly.

As for stretching, as long as they aren't over-tensioned, you won't notice any stretch duting the time of a FIRST season.

As for belts for non drive base use, I say go for it, they rock because of weight and size.