Unless you have very tight clearances, 25s should be okay. However, it depends on the 'actual width' of the tire you are mounting. Is your 28 a true 28mm when inflated on your particular rim? What rim is it, by the way? The wider it is, the wider the tire will measure.

As an example, I've got 25 Michelin Krylions on Open Pro rims, and they measure 27.5mm wide!