Bike Direction on Roof
Mainly it doesn't matter.  Usually two bikes are front-first (because it "just seems right"), but once you go beyond two bikes the directions alternate.  True, if you don't have the bike properly fastened then, on a fork-mount carrier, it can blow loose a hair easier in the reverse orientation, but it can also come loose going around a curve or going over a bump, so you need to be sure it's well-fastened in all cases.

Note that you may find, based on whether you're right-handed or left handed (or maybe you have a bad shoulder or such), that it's easier to lift the bike onto the roof one way vs the other.  In that case it makes sense to have the bikes front-facing on one side and rear-facing on the other.  OTOH, the shape of the vehicle, position of the doors, etc, may factor into your orientation decision.