Keep a good eye on the situation in which you're fighting.
If your target is near to their own graveyard or is just mounting, the approaching direction generally doesn't matter; your target will be sitting down relaxing and cursing about that overpowered warrior that just kicked his ***, he won't see a rogue coming (providing he didn't see you stealth 30 yards away). On the other hand, people who are in the process of mounting up will not be moving towards you while you move towards them, lengthening the time before you can get off a cheap shot. This is something to keep in mind as well.
If you're not too far from a big fight, your target will be on it's toes. Especially without improved stealth, approaching from the front will have a good chance of your target seeing and hitting you before you can get an opener of.
If you're jumping into the middle of a big fight, your approaching direction usually does not matter since the target will have somebody else targeted and will be too busy running about and fighting. The chance that he or she sees you in time, notices, and manages to get you out of stealth is extremely slim. However, remember which enemy you're facing. Every class that has an instant AOE (mainly mage and paladin) should be approached with caution; if your target in question (for example a mage) is running away from the battle to gain some distance for a spell or bandage, there's a good chance he or she will throw out a simple AOE if you approach from the front.
Overal you should try to head for the rear whenever possible, it severely lessens the chance of them discovering you. There are just some exceptions. The best tip I can give is the one I opened my post with; keep a good eye on the situation in which you're fighting. If your target has just killed somebody he might be distracted. If your target was moving towards a big battle and suddenly stopped there's a chance he is typing or doing something AFK. Take note of your surroundings and (ab)use your knowledge.
On a small sitenote; all this is BG-related only. If you're doing a 2v2 match in the arena, you should expect your enemies to know there's a rogue there; it's not that hard for somebody to figure out that a player is missing when it's 2v1 in a 2v2 fight.