I would say that you'r just fine in the 70's.
I just find it strange that your laptop went up to the 90's, and now for no reason has "fixed itself".
Well, anyway, if your laptop stays in the 70's it's really ok, even if it gets to 90 on havy load it's probably ok for a laptop. But you should worry if again you notice a cpu temp difference with and without the power plugged in!