+1. Watered down beer sucks.
Sorry to revive a dead thread, but I thought I'd add my $0.02 in case anyone reads this later. I've topped off my beers at all the different stages (during primary, during secondary, or at bottling) at one time or another, and never had any ill-effects that I could tell.
I recently made a special bitter with an OG that was too high (1.052, diluted to 1.040), and topped that off with water at bottling. The beer definitely tastes watery now, and it's bumming me out. So I would say use caution when topping off low gravity beers, and don't worry about topping off higher gravity and fuller-flavored beers.