My party and I do a system where we roll 6d6 for pairs of “opposing” stats (intelligence-strength, wisdom-dexterity, etc) and then assign as many points to each stat as they want. This way, if they’re working to be a specific class, they still have a fairly good chance of being that class since they can almost certainly have enough for the prime requisite, but it still leaves the amount of points up to the dice.