I normally try to get hold of the better quality primary drive gaskets which are a bit thicker. Some of the cheapo kits do tend to have pretty flimsy paper jobbies. Unless the mating surfaces have been damaged, the primary drive joint is not normally prone to leaking (the gearchange O ring is the usual cause of any problems from this area). If all looks good I usually just grease the gasket lightly. If in doubt I use something like Wellseal on one side only so that the case can still be removed without destroying the gasket.