I have had my Cooler Master 212 Evo running on my current system for about 5 years. It has been running fine and I have not done anything to it since I installed it other than dusting. Is it safe to re-use this in a new build that I am planning to make this Christmas? Or should I just buy another cpu cooler?
Totally "safe". The heatsink element won't degrade and, as long as the fan still spins, doesn't make any strange noises etc, it's fine too. Even if the fan were to fail, it's easy enough to replace.
Saying that, depending on your new system, it might make sense to just buy a new cooler.
A 212 Evo was the budget king for a long time, but it's not the best cooler out there... and not even the best cooler in it's pricepoint anymore.
I'm on my 4th build with the same scythe mugen rev1 12 years old.
Not the best cooler but it works like a charm. Several adaptations for the unsuported sockets, improved the mounting with some machined parts for it, changed the fan a couple of times. Going strong on Ryzen 2600x atm (started from E4400 in 2007 on lga 775)
So if you are happy with your thermals and it's easy mount on new system, go for it.
A good aircooler is never obsolete. I keep at close hand older coolers like old Thermaltake Volcano on SoketA, and other really old coolers for diy NB cooling or VGA cooling or other cooling mods.
I might mention that lapping the base of the cooler on a flat surface is good twofold: it flattens the base and also gives a shine and polish dingfree. It also ansures a good thermal transfer. Old coolers tend to darken the base with okides on copper and no amount of cleaning is going to make it shine again.
(depends on the model. if the heatpipes are on the botttom.. better don't so you don't risk a hole in the heatpipes, but if it's a solid copper plate, go for a lap if not shiny)