That sort of depends upon what your ambient humidity is to start with. If your home is air conditioned, for instance, the indoor humidity may be pretty low. Lighty misting a clean viv is never a bad thing. You can increase the humidity by putting a cloth (towel, sheet, shirt, whatever) over part or most of the screen/ventilation area.
If a water dish is on the warm side, evaporation adds humidity to the air. I clean and change the water daily when it's on the warm side.
A moist hide is also a good idea when the snake is shedding. That's a hide (margarine or whipped topping tubs are cheap and easy) with something damp for the snake to squish itself into. Sphagnum moss is ideal, but cotton balls that you sort of pull apart can work in a pinch.
Personally, I mist the viv, and cover it with a towel when I notice a snake's in blue. However, not all snakes are interested in a moist hide - and I have missed the blue phase when I first got my snakes. Only one has had any problems at all with her sheds, and that was when we first turned on the AC for summer. She got a moist hide and extra misting for the next cycle, and all was well.
Hope that helps.