Istari are expected not to cheat, and to promptly report bugs which could be abused. People on the Istari Quest must not commit evil acts: the slaying of a hobbit child will affect a Metamorph, Elda or Istar far more than it would affect a normal mortal. Nor should an Istar deal in evil items of power, such as mottled cloaks or morgul blades: an Istar should destroy similar items, not use them or encourage their use.
Since their main task is to help and protect newbies, Istari, Eldar and metamorphs may not engage in professional orc/troll hunting. Defending Fornost or Bree from an onslaught of Orcs is fine, also encouraged, since it defends the young people of the West. Traveling east with legends to hunt them down in their lairs is not. The job of the Istar is not to seek his/her own personal fame or profit, but to be a source of help and knowledge for the inexperienced. Unless a young character wants your protection on a journey to Rivendell, there should be little reason to be found in the eastern regions.
Murdering an Istar or the Istar's pupils will cause the murderer to be put on Sauron's List. On the other hand, Istari who follow Saruman's path to evil will probably share his fate.
See also: Istari