Really, the only time to purge is immediately after kegging in order to dilute the air in the headspace or maybe to drop pressure in an overcarb condition.
Rearranger IPA - Warrior, Simcoe, Lots-o-Cascade
Ponchy Pale Ale - Millennium and Willamette
NippleRub Ultra (Light Lager w/Mexican Lager yeast)
In the Chamber:
Rearranger IPA - Chinook, Simcoe, Cascade