For both CPU and GPU I will get the highest stable OC I can get with minimal or no voltage increase, and then leave it there.
Back when I had my R9 290x I got lucky, I managed a 1080mhz core overclock (1000 stock) and a 1400mhz memory overclock with zero voltage increase and only a 15% boost to power limit. It stayed stable without thermal throttling for about 6 years until I retired it for my 5700. It now lounges at stock clock speeds in my HTPC in the living room.