There sadly is a big world outside the DIY market. Having used a now-secondary C2D build for 7 years, with a stock cooler which never was cleaned, for hours (or days at stretch), without any issues, I'm quite sure the performance doesn't degrade. How do you push a system to it's limits?
Prime95, done.
Hours of video encoding, done.
Days of usage, done, alot.
Heavy multitasking, been doing for years.
Anyways, as per the original question:
"I am using an Intel i5-2320K. I am thinking of buying cooler master 212 evo for my CPU.Is the stock cooler enough or is an aftermarket cooler necessary?
I will not be overclocking my cpu anytime sooner"
No, OP, af cooler is completely optional.