In a sense we shouldn't always make laning dependent on the Jungler, however, in what situation and what lane should a Jungler prepare to gank?
Situation? Example: If the enemy is pressuring a lane to allied tower, however your allies are low health; or bottom lane is having a bit of trouble but enemy top is out of position.
In general, ganking is most opportune when the enemy is over-extended in a lane, so you can more easily get a kill. If you run around trying to gank, but don't landing any kills, you'll quickly get underfed and have a hard time throughout the game.
If a lane is struggling, you can have them let the enemy push to set you up for a better gank. If it's too easy for them to get away from you, then your gank is pretty much useless. That being said, in early levels, if you can get the enemy to burn summoner spells, that can be considered a success.
Each jungling champ is a little different due to skills. As Zach mentions, just getting to burn summoner spells pre level 6 is pretty good.
After 6 most junglers have good ults that make ganking much better and in turn allowing for better ganks when the enemy is not over-extended.
For example, I often jungle with Skarner. He's pretty useless until level 6 for ganks unless the enemy is over-extended for low hp. Master Yi on the other hand can help to push a lot of dmg pre level6.
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).
When the lane is pushed to your tower and the enemy champ is at your tower, and when the enemy is low in health.
