I'm assuming you mean that in the case of a partial boil, you would pitch into the top off water.
I wouldn't, and here's why:
Top off water has a pretty low concentration of ions (might be hypotonic, anyone know?), and isn't an ideal medium for brewers yeast. It also might be pretty cold for the little critters.
Otherwise, yes, it would help mix it in, though they'll do a good enough job of mixing themselves in on their own.