What color(s) are you going to be painting? If they are the same darkness, you might be able to get away with a single coat. But if they are lighter, or worse you want the similar light shade on both the wall and ceiling, you're going to have to prime most likely.
Look at it this way, even if you don't have to apply 2 coats, you'd probably get a truer color with two coats. Would you rather apply 2 coats of a quality ~$30-50 paint? Or would you rather one coat of a $20 primer and $30-50 paint? You're time is going to be about the same either way.