Telecanter’s Receding Rules has an excellent post on wandering monster checks, covering the different ways in which various old-school rulesets handle wandering monsters: how often the DM checks for them, encounter distance, increased probabilities if the PCs are making noise or loitering at a major intersection, etc. Telecanter also offers his own house rules for handling wandering monsters.
The details of wandering monster encounters are important for the old-school DM to consider. I’ve been using the Holmes rules (roll once every three turns), with additional rolls when the PCs do stuff to attract attention. Now I’m considering moving up to the probabilities in Moldvay (roll every two turns). Rolling every turn, as in OD&D, seems too often to me, unless it’s to make up for absent-mindedly forgetting to roll half the time.
What method do you use for wandering monster checks in your game?