The process of getting and S on a champ involves everything that happens in the game and compares it to how others did on that same champ in that same role. This is to say that yes assists can mean a lot in jungle, but it all depends on what other players did previously. If you are say Master Yi for example who on average has more kills then assists in a game (Yi's the average being around 8-9 kills, 6-7 deaths, and 4-5 assists), then if you go 1/6/17 you may be able to get an S, but it will be more difficult then if you just wen something like 10/5/6 where every stat is better then the average. Obviously the example is not all inclusive because it also takes into account things like total gold earned, items built, level, cs, etc.