I get the occasional slime, but never any bad baddies. It's well lit when I'm growing trees, but since large mushrooms have to be so many blocks apart (5 for red, 7 for brown) I had to remove a bunch and stagger the light sources. If I were going to alternate between trees and shrooms all the time I might install redstone lamps with switches depending on what I'm growing, but for now torches are fine.
As long as the torches are placed 7 spaces or less from each other I don't seem to have a problem. That is, i-------i, but that's really only good for rows like plantings or skywalks. You couldn't light a field like that. It'd need one in the center of the 9x9 square.