I let the dice fall where they may (1st level, however, gets max hp). I do the same for my main NPCs. Over time, this tends toward being indistinguishable from average hp anyway.
I don't think that rolling and letting the dice fall where they may makes a mockery of different hit dice types. If that barbarian rolls a couple of low hps for a couple of levels, then he had darn well better be sure to learn to be a survivor. That's the bottom line. Adapt or die.
We've had wizards with nearly as many hp as the front line fighters, sure. I don't see that as a drawback for the campaign or the game. It does mean that the party has to be more judicious about the fights it picks and how long it fights before finding a place to rest. It also means that the fighter will almost certainly be pressing the wizard to make him an amulet of health...