I've found that most things wear out in pairs anyway. And if you're going through the trouble of doing one it does make sense to do the other... however, this rule only applies, for me, when doing something that's just "had it's day and needs replaced"
Things that I broke... because of some change I made or just me hammering on it too hard, I would likely only replace the failed part.
For endlinks, it would depend on why it failed. End of life, replace both. If broke due to abuse, just replace the one that broke.
If broken several... may want to re-access the bigger picture.