How do you approach a 3-4 stone?

[Subotai]

When you approach a 3-4 stone how do you determine whether to kakari on the third or fourth line?

I for the most part always go to the fourth line as I am more familiar with those joseki.

Is there any principle that can help you dictate how to kakari the 3-4 stone?

[DrStraw]

Balance. You don't want all your stones (or your oppoenents) on the same line. But you need to learn a few simple low approach joseki so that you can evaluate which may be better in any situation.

[Kirby]

Of course, 4th line can lead to more influence, 3rd more for territory.

But also, there are a lot more joseki for 3rd line 3-4 approach, so it can be more complicated. High can be complicated, too, but has fewer known joseki variations, so in some cases is easier.

I think 4th line is more popular in modern pro games, maybe for both of these reasons.

[skydyr]

In general, though there are joseki that go the other way, the 4th line puts a little more emphasis on the side that the 3-4 stone was originally pointing and on center influence, while the third line puts more emphasis on the corner and the originally weak side of the 3-4 stone. The 4th line approach is also somewhat lighter than the 3rd line, so when the opponent outnumbers you, a 4th line approach (or a distant approach) may be more prudent.

In a sense, which way you approach can also depend on whether you want to treat the original 3-4 as a third line stone, and cap it, or as a 4th line stone, and aim to undermine it.

EDIT:
One other thing to consider is that if your opponent tenukis, would you rather have a 5-3 joseki or a 5-4 one to continue with? Both have their virtues, of course, and the applicability of this depends on how advisable tenuki is. If there's miai, as with another unapproached 3-4 stone, it seems quite possible, if it's the last unplayed corner, maybe not so much, depending on the rest of the board.