IMO, it depends on what you're after...if you just want any old powder to prime with, maybe out of the same source as your main powder charge, then just use that approach.
But for me, the speed of lock time is everything in a Flintlock, and we know that optimizing multiple elements of a lock's operation add up to collectively increasing the overall speed of ignition.
Just like a properly tuned lock, and having smooth low friction internals in a lock, and using very sharp black english flints contribute to faster lock time, we also know that 4F priming powder is faster than a larger granulation...it's why 4F is made / sold as priming powder.
So with best possible lock time speed as my goal I prime with 4F, along with using sharp black english flints...same as I try to ensure all other lock operation related activities are geared towards best possible speed.