Should I paint the exterior wooden trim on my home if it's peeling?
If the paint is peeling, then it's time to repaint.
You probably could get away with touching up the areas where the paint is peeling, but it's difficult to get a good color match, and it's probably almost as much work to do touch-ups as it would be do redo the whole thing. If there's a scrape in new paint after only a year or so, that would probably be the time to touch up just the scraped area...
Prep is the key to a good paint job:

Sand and scrape off any loose paint.
Caulk any small gaps.
Repair any large cracks. (Ask a whole new question if you're not sure about how)

Enjoy another 5, 10, 15 years of nice looking paint, then do it again.