Don't gank the lane that needs it, gank the lane where you're most likely to succeed
So, hypothetical scenario is this:
Bot lane: Blue Taric Ez getting owned by Purple Lucian/Thresh. Top lane: Blue Xin vs Purple Karthus (always pushed) Mid: Let's keep mid out of the picture by assuming they're equal. Jungle: Blue Yi and Purple Kayle (to make them somewhat equal as both farm junglers with shitty ganks)
Xin, as one can imagine, would theoretically have a very easy time killing Karthus on his own, especially early game. Blue bot would likely be spam pinging for help. However personally, I think a 0/0 Yi would probably wouldn't do much at bot anyway, except maybe scaring them off to heal after burning his ult/summoners, accomplishing nothing. However, if he camps top to share some of Xin's income on Karthus, and then goes bot (maybe with Xin's help after owning top and taking turret) after getting a few items to even the field... what do you think?