Farm goes first always!
when the jungler comes to gank, then you go for the kill
when you wanna roam the map if you are mid for example, push the lanes asap, dont care for the farm you get, but try as many as you can and then go for the roam
and when the match reach the teamfights stage, then make sure you never leave your team, unless there's a huge wave somewhere, but then you should inform your team instead of running away without noticing anything