you know some is normal, so you are having more then normal? if all your plugs look good then i would say it is normal, til they start getting funky.
that aluminum engine had normal looking plugs but the piston to cylinder clearance was excessive for an aluminum block and i had really lightened the tension on the
oil control rings and probably washed the cylinders pretty good when i was messing with the efi. this engine would do the same thing with puffs of smoke out the crankcase, but it still ran good. i would say your dilema runs somewhere in there.