What you don't want to do is tape them, you'll never know whats going on under the tape. really small nicks and fraying I melt and smooth out, but there is a limit to the effectiveness of that. If it looks like the strength of the rope may be compromised, cut it and keep the longest piece. If its a climbing line and its pretty wore out, switch it out to a lowing line.
You have to weigh all the factors, the price of rope vs the price of rope failure. There comes a time when it just makes sense to retire that sucker and buy a new one. Might seem expensive, but compared to a new roof or your life its cheap.