I've just recently had a bathroom redone and chose light grey tiles. Like you Button, the salesman reccomended I use darker grout. I wasn't sure about this, as I thought it would look odd, seeing as I'd be using white grout on the walls.
Anyway, I decided to take his advice and chose light grey grout and it actually looks very nice. On white tiles though, I can't really visualise how it would look, light grey would probably look quite nice.
In my own bathroom, we have large brownish tiles, the colour of business envelopes, we went for cream grout on both the floor and walls, but we have identical tiles on both. I think if you are using the same tiles on walls and floor, then it would look better with the same grouting on both.
Regarding the grouting getting dirty, well it really depends how you treat your home. We don't wear outside shoes upstairs, so any dirt is probably dust, which is easy to clean. We did our bathroom a year ago and the floor grout still looks new.