The only reason to use a 25mm tire is that you want to use a lower tire pressure, but still avoid pinch flats. According to Sheldon Brown's web site, people who claim that they have lower rolling resistance with 25mm tires are overinflating their tires. The proper tire pressure for a 25mm tire is lower than for a 23mm tire, thus giving you more cushioning, but also a small increase in rolling resistance (as well as weight).