I have run an 11-28 or a 12-28 cassette on the 2 wheel-sets I have at home. For those two switch-overs, I know I don't need to change the chain length, and the rear derailleur adjusts (right?).

I need to get a new cassette, so I'm wondering if I move to a 11-25, will I need to alter the chain length if I switch between that and the 11-28 on the other wheelset?

I have a race coming up that has almost no climb at all. Was wondering if the 11-25 will be a better choice for such rides, with availability of greater gear selection in the middle...