Good question. If Im working with a good character/team then stealing by default is prohibited. Unless the story, conditions warrant it. In ad@d Im typically neutral good alignment, which tends to carry over to just about any rpg.
If my character/group happens to be evil, well it all depends. Often items available for stealing particularly at the start or early levels of the game are transparent options for power gamers. As such while their may be no ethical quandries for this course of action, extraneous considerations (e.g. challenge) may inhibit these activities.