If an exchange improves your opponent's position forget about it. This is murky territory and one must read lots of books and/or study lots of games to obtain the correct feel for exchanges. Ulf Anderssen is exceptional among GMs as to what to exchange and when.
It's not as simple as knight takes knight doubling pawns, since your knight's activity was likely more valuable than a new "weakness" you can't really exploit (the new semi open file could even be useful for him) and if NxN on his end he'd weaken valuable points near his king and help you activate a rook or queen. This is very general obviously, but does apply in practice often.
It's often said that maintaining the tension in chess is good rather than trading when pieces are under fire. However, there are times when it is obviously good to take also.
What are some principles for thinking about when to maintain the tension and when to take? Illustrative examples would be great too.