From: ffplayerVII People say the greenroom has more Tonberry encounters, but I think it's all bull. It's all based on luck.
Well, the tests (although not very extensive) do show that the green room and the back room do spawn Tonberry at a higher rate than the other rooms. Either way, it's worth a try. I've always gone to the green room in all my playthroughs. Works for me.
"We aim to please. You aim too, please." -Janitorial Staff
Just try to make the best of it. Don't think of non-tonberry encounters as a bad thing, I always Mug while capturing, and you can get your hands on some decent items. If you encounter any overstock fiends, have Yojimbo kill them off to help raise his compatibility. Magic Pot encounters are a pain, but I always take the opportunity to see how many Bingos I can get.
Either way, if anything, stay near the back of the cave, save and reload your game every dozen or so fights if you don't see any toberries.