i always answer these questions thusly : "tork it up till it strips or breaks the fasteners, then, back it off 1/2 a turn. "
however you feel about this answer is ok by me.
in reality, on a valvecover gasket, i tighten those carefully, whether they are factory tin, cast, billet, or fabbed. i use a 1/4" ratchet with whatever is needed to access the retainers, then snug it good. observe it some, then go over it a few times until it doesn't weep, and stays dry.
what this does [for me anyway] is ensures the flanges don't get distorted, and you don't need gobs of sealant on the head side of the gaskets. this is just what i have done over the years, on all makes and brands of engines, and have found something that works for ME.
lots of guys have come to me asking to fix cracked or bent flanges, or remove broken bolts from the heads because of excess tightening.
do whatever works for YOU, but it won't hurt to sneak up on it instead of "crank it down till it strips or breaks, then back it off 1/2 a turn"