I rode 25s all season: Conti 4 seasons, then GP 4k. Dropped the pressure a bit for the roubaix races. It worked for me. (normally ran 90 in the front, 100 in the back) Dropped it accordingly to the surface. And I run pretty standard rims.
A buddy of mine got 28's for an especially sandy roubaix around here and said it helped.
One of my teammates gave me crap for all of the extra rolling resistance that I'm creating, but honestly I can't tell the difference. If you buy
Zinn's argument, wider tires have a lower rolling resistance. The only drawbacks for me are adjusting the break pads and (in theory) more aero drag. Considering I'm racing on an open pro and a Klinen rim, I don't care about the latter.
Flat protection has been much better with the 25s. I didn't pinch flat all year, and only one puncture that I can think of *knock on wood*.
Also I find the conti 4 seasons to be comprable to the GP4k, however it does wear quickly.
Go for it.