just by the way the guitar sits on the left leg you get to play the higher frets very easily, and the right leg lets you play the lower frets much more easily - think about where your left hand naturally rests, and it will be obvious that the left leg helps you out a lot for mid-to-high frets. The wah wah is always right foot, but a prop stool (the tiny 3-inch one for classical) is always left foot. I like a real stool with a cusion on top, about 1 foot or a bit more high, rest your foot at 45 degree angle against the edge of it (cusion gives grip and is best with good solid comfortable shoes on), and you are locked in place with your guitar, you can get really good control...but it sucks having to have a stool like that all the time...
(without a stool) crossing your left leg so your knee points outward is a trick of the guitar trio, it also helps to lock the guitar in place because the base of the guitar will rest against the inner bottom of your left calf when you put it on your left leg...but this is uncomfortable for me - my legs have an uneven build and always get sore from that kind of thing (it's good to switch I guess...if only to experiment with what feels stable for you). If you cross your right leg, put the guitar on your right leg, otherwise it has nowhere to sit if you have your right leg crossed and try to keep it on your left leg - it just tries to slide down your leg.
For electric, neither leg is that essential. I guess the right leg would be the standard for that - I don't really pay much attention with the electric, especially my Ibanez with such a slim profile...but I like how heavy the Ibanez is, that helps it rest in place also...
...and last but not least, a chair or stool with cross-members (the wooden or metal sides that you can rest your heel on) can help too...so try all of these things out and see what you like...