Depends in part on what you watch the most. Here is one rule of thumb I have seen discussed in the forums, its about right for us and it might work for you:
HD 3.3 x Picture Height (PH)
DVD 5 x PH
NTSC 7 x PH
My RP widescreen 55" Picture Height is about 27" so the recommended viewing distances are approximated being:
I sit about 11-12' for most things and find it fine and I can move closer for HD if necessary. NTSC is a bit ragged but its not too bad. Good luck.
[This message has been edited by bdenman (edited 08-14-2001).]