I try and always do a preview render at smaller resolution just to check and make sure there are no serious 'gotchas'. Also, a preview render can often provide a clue as to how long a finished render may take. IOW, if a preview render takes forever, then so will the finished one.

For large renders, I sometimes set atmosphere to Global Ambient, and move way down the sky dome quality (-2 or lower) and do a 'final' render at lower resolution to check on details.

Sometimes, if I use a huge proc terrain or large hypertexture, it can take 20+ hours on a quad core system-- in those cases I just use my laptop for other work :-)