Thats an old BMX racing trick that works well on dirt so has been adopted by MTBers. I don't see how having a larger tire in front would be as beneficial on the road, since traction is less of an issue. It would let you run lower pressure in front, making the front end a little cushier, and make it easier to navigate road irregularities without increasing much rolling resistance. However, if you want bigger tires for those reasons, I'd probably go with larger tires front and back. The speed difference between 23mm and 25mm tires is probably not noticeable to your average amateur, but the overal handling differences are potentially significant. I use 23s and 28s on different bikes, and the 28s are way more comfortable on rough surfaces and handle bumps, gravel and other irregularities a lot better. They are a tad slower on smooth pavement, but are actually a bit more efficient on rougher roads. The nastier the road, the better a larger tire will perform relative to a skinny one.