For a sprint it's optional but I personally do eat a Gu on the bike. Just take a piece of electrical tape wrap it over the top tear way section of the Gu and tape it to your top tube on the bike that way you can just reach down and rip it open. For anything longer then a spring I would defiantly say that eating on the bike is a must, and if you do long training rides good skill to practice.

I don't tend to use them in sprints (unless I'm making sure they agree with my stomach when going hard as a test for another race). But some people find they need fuel sooner than others -- I'd try it on a training ride and see what you think. Remember the big rule: nothing new on race day :)

oh yeah and one thing I forgot to mention on my first post is when you wrap the tape (also do this your bike number) is make sure you thread it under your brake line and not over the top of them. Not sure how much impact it could make but I always see all the pros and top finishers at the tris I do it, so figure some logic behind it.