What i meant by increasing 5-10 and then testing was just for stability purposes and specially since it is your first time. you are supposed to keep going until sometimes goes wrong during stressing. eitther stuttering, artifact or crashes.
after that you have to lower down to a safer / stable overclock.
50+Mhz are what you are looking for, but it depends on every situation. sometimes you are stuck in 55-58 FPS and Overclocking the card gets you right there in the 60FPS+ sweetspot.
edit: also, begin overclocking Core Clock first untill you reach that limit. then if you really feel like to, then OC the memory but you will not get any noticeable performance improvement untless you get 250+Mhz.