Scary, *awesome* tip -- I have to try it!On my last run through Bane (almost a year ago), I started a project around this very issue. I was trying every combination of left and right, multiple times, and writing down the groups of monsters you'd find in each section of the temple. I didn't finish my experiment, but I seem to recall that there was some variation (IIRC, you could find mind flayers both on the left or right paths), but once you went to an area and the encounters were calibrated, they would NEVER include monsters outside of that "set". But, I think if you reloaded just before you opened the gate of a particular section, you could generate a slightly different "set" of monsters in that area. Again, I would have to look at my notes or redo my experiment to give more accurate information.
However, the real issue is with greater demons and pit fiends, which are never found together. Nothing else drops anything overly important, except priests of ramm (which are always available) and keepers of the crystal (if you have a psionicist). According to the treasure table I have seen, they *should* drop the same set of "best" items, but given the rarity of GD/PF encounters, and the rarity of the special drop (2% I believe), it is VERY hard to determine if GDs only drop some of the items and PFs others, or if one is more likely to drop a particular item. But using Scary's method, you can try both in the same game without editing keys! (Also, I think accursed ones, found very rarely in or near the dark forest, can drop from the "best" treasure list -- I found a zatoichi bo this way.)