If done properly Overclocking is not harmful to your system. It is a little bit of free performance at the cost of heat and power.
If dont improperly damage can occur, but that is rare, and only in cases of people not knowing anything about voltages.
You can OC if you want to, however a 4790k should be enough for most needs at stock clocks. If you dont need more performance you dont need to OC.
I also dont recommend OCing on the stock cooler.