Would you say if the lanes are pushed? If I notice the opposing jungler in one of our lanes? What is more important?
You counterjungle in several cases:
1) You know you can take their jungle without running into anyone. This happens most frequently when enemy jungler has just died or pops up on the map in another lane, giving you a window of opportunity to manipulate their jungle.
2) Your jungler/team is stronger, giving you the confidence to win fights even if you do run into someone. Examples of strong junglers are shyvana, udyr, and skarner.
3) If your opponent is absolutely dependant on certain buffs, like amumu on blue. You also contest their blue buff if their mid (like anivia or ryze) really needs it as well.
yes, as a jungler, you want to counter only if you know you can take what you want without any incident, OR if you are certain that you can kill whoever is going to contest you. The optimal time to counter jungle is when their jungle is ganking, fail ganked and died, or ward their jungle so you know when their buffs/jungle camps are up so you can kill them and keep the spawn timers in your head. Regular jungle camps are 1 minute, Buffs are 5.