It all depends on what you want with the print. If it is just to hang on the wall like a painting high detail all over will only distract from the important parts. Select your detailed/sharp areas carefully so they stand out from the background. That is how a traditional painting is made. One starts wide brush and then goes down to improve selected areas with finer strokes.

In Poser terms you would probably do an overall render in a moderate resolution and then in post process patch with partial renders in higher resolution where you think necessary, and use transparency and blur to match.

You could even consider to leave out time consuming details (hair) in the global scene render if you know you will patch the area later anyway.