This is one thing that bugs me: If the rogue finds a certain amount of wealth in a chest, how much is considered "bad form" or "impolite" to skim off the top? 10%? 20%? 10gp? Enough to make the remainder divide evenly? If the rogue finds a hidden treasure, like a ring buried in muck, is he supposed to share it with the rest of the party even if they had no hand in finding it (and in that case would even a good-aligned rogue keep it for herself?)
I think that stealing from another PC is where I'd draw the line of "gone too far". Stealing from random NPCs can be a nuisance, skimming off the top of a treasure hoard shouldn't be noticed, but stealing things from other PCs sows distrust and irritates the players, which tends to make the game experience less fun for everyone. And if someone gets annoyed that a rogue (whose sole reason to go adventuring is probably to get rich quick) takes a bit more than their fair share of the treasure, that's when the DM starts telling the rogue what they find via note.
It's hard to say. Some people have said that the rogue's class name was changed from thief to rogue because calling it thief before actually convinced many players to steal as such, regardless of alignment. Then there are Kender.
So you have a lot of stealing going on in general, sometimes light hearted, and other times just plain selfish. The wise thief would be pocketing as much as he can while neither giving himself away or dissuading the rest of the party from penetrating deeper into the dungeon, where the thief knows more riches are likely waiting.
A good-aligned thief or rogue would probably draw the line on taking so much of the treasure that the other characters aren't really being rewarded for their efforts. A neutral thief would only care that the other characters are not deprived of any necessities, so they should get enough to get by. An evil thief would not care about them at all in this way, but instead would judge them by their worth to him. Finding a magical sword or wand, for instance, could go to the party's fighter or wizard instead of being sold or used by the thief, if he felt they could make the most use out of it. The thief would judge the party will get more treasure faster if the fighter has a better weapon, and if the wizard doesn't have to spend as much time preparing new spells.
And a lawful thief might abide by a contract he signed, even if he is evil, but that is only "might". It would depend how likely he thinks the other party members would be to abide by it and by other contracts in the future. If he is dealing with chaotics, he will not want to put himself at any possible disadvantage.
As for stealing from other PC's directly, good-aligned Kender can be forgiven, however annoying they can be. They will offer up the items or money taken if they're needed, and innocently ask if you're looking for this. (They really don't consider themselves thieves when they take things.) Neutral and evil Kender, needless to say, can be much more of a problem and may need to be exiled.
I recommend everyone keep their character sheets inside folders, and only share information with others they need to know. The DM, meanwhile, should have copies of these sheets in print or in electronic form so he can mark off whatever is stolen without the player knowing. This is important even if you choose to disallow inter-party stealing, like most do, because monsters and NPC's may be lurking about and the players shouldn't always know if they've been robbed or of what. (Taking their sheet to erase something kind of gives it away, LOL.)
Passing notes between player and DM is a useful system, and so is stepping away from the table briefly to have a chat. In my campaigns, I have seen players who were rivals in all things in life, and they brought their feud over into the game. They were quite a spectacle for the others, and sometimes every ten minutes they would be threatening each other. One time, one of them stole a Gem of Seeing from the other player, and I didn't have a system like this in place and let me tell you I heard from the player what he thought of my allowing the theft! So it can be really hard. The player took the gem, which was like the other's most prized possession, and he was a guest in his campaign. He teleported away with it, and the other swore to hunt him down and get it back.
So, you see, it can be some degree more painful if you allow any sort of this behavior at your table. I do not recommend it, but it can work and it will much better if it's better managed.