now see to me this is where trickiness comes in.
first off, HOW does a ally influence whether or not the jungler would come back for a gank. For one communication. if we ping on our way, and we go in, we'd want you to go in with us as well, not just farm. if you're oom or oohp, ping it so we know.
Secondly, we are willing to go back to a lane if they do assist us and its a successful gank (getting a summoner or kill for example) if your laner co-operates with you and join in on the gank instead of cussing them out or spamming ? on them if you die, we're more likely to come back to the lane when you ping assistance.
as for behind lanes? it all honestly depends. 1) who is the enemy? 2) how far ahead is the enemy and how far behind is my ally? if say its a fiora or garen who's 2/0 at 7 minutes and my allied top laner does help me out and we were close to killing them, then i would try to come back. they arent too terribly behind, and they are willing to help with the gank.
Now say the fiora or garen is 4/0, and you're allied top laner is 0/4 and dies from solo kills. that makes the lane a little bit more harder to gank imo because now the enemy top laner is fed, and with knowledge that she can kill solo, she can fight a 2v1 and escape. So it would be harder to gank that lane, and try to not feed her anymore. If it gets to the point where i try to gank that said lane and now she's 7/0, to me, that lane is dead, and i cant gank that lane and i'd need help from say mid lane for example, or else that said laner would be even more fed.
But all and all, i would gank where the lane is strong, but not heavily pushed, since most of my junglers aren't the best towerdivers. but if they arent TOO far behind, id try to help out. if its a successful gank, they communicated with me via pings, and assisted me with ganking their lane, i personally would come back later on.
that's just my opinion anyways. im just a unranked casual player