I take my male platy fry out as soon as their gonopodium begins to develop; lengthening slightly and just beginning to change shape.
Inbreeding is best avoided, guppies and other live bearers are usually already inbred to a degree, but the more it occurs, the more problems you end up with. Guppies are supposed to be hardy fish, but now so much of the stock is weak due to inbreeding. I know they purposely inbreed every now and again with certain fish e.g. bettas. But allowing your guppies to inbreed has no real benefits.
I consider attempting to avoid inbreeding in my platies as part of my responsibility as a fish keeper