General rules of thumb I use:
1) Push when there is no one defending a tower, even if you could get a gank in an adjacent lane
2) If you have a teleport ready, gank when the numbers are equal elsewhere. I.E. if it is 2v2, jump in and make it a 3v2, this will allow you to push when the 2 opposing are dead.
3) If you are outnumbered in your lane; even if you don't have a teleport ready. Either move to the nearby one and gank or fallback to the nearest friendly tower. This depends greatly on where the other teammates are or how far along their spawn timers are.
In essence, the gank is a tool to get to push lanes.
Of course there are more minute details, but that is the gist of my strategy.