Thanks for the replies. From an engineering perspective, traction should increase with increasing load based on force vectors and friction coefficients, yada yada.
What I was curious about is from practical experience, does the front tire in fact continue to stick better the further up the seat you move? To the point where the back end breaks away. Sounds like the answer is yes. The reason I'm curious about this is often times when the front end goes away on me, I would have sworn I was far enough forward.
So I guess the bottom line, if you're losing the front end, you need to move up. It's good to have simple things to keep in mind for late in a race when you're tired.