This should spark some interesting opinions.

The rule of thumb passed on to me was that as soon as the ammo and nitrites drop off and nitrates begin shooting up from the cycle, that's probably the ideal time to fire it up. WIth the reasoning behind it is that you want the ammo and nitrites to actually have the time to do their job to start as much of the good bacteria as possible before you start pulling stuff out of the water column.

On the other hand starting to skim as soon as you start up the tank will keep the water clearer too and possibly eliminate the nutrients for algae growth too....

Good question!