Putting aside specific champions. There are a few general situations where all junglers will probably make the same decision. I won't really detail things such as pushing up lanes for lane ganks bot or knowing where wards are (this should be common knowledge).
For example, your top lane is 0-3 and is simply being outplayed by the opposing top-laner. Generally, the decision here is to snowball your mid or bot lane (most probably mid because the impact is faster and more significant) and completely ignore your top lane. Being ahead 3-0 in a top lane is a ridiculous amount and almost always impossible to overcome without a gaping error by the opponent.
Another example, you were counterpicked in your top solo lane (let's say Olaf vs. your Warwick). It is inevitable that WW will be losing his lane early on, so your jungler decides to gank early and often in order to keep Olaf in check. Ganking is not solely for kills, it can also be used to ease pressure off of a lane and let them farm.
Bottom lane is a different story due to the support. Ganking through river or tri-brush should be near impossible against a competent bottom lane. Lane ganks through brush are then the only option (unless you're Nocturne or possibly WW/Rammus). Ganking bot is either a result of nothing else to do because your jungle is cleared and solos are pushed/fine or you are trying to snowball your bot lane (see: MF) or help a weak early-game AD (see: Vayne).