If you are buying NEW wheels to go with the wide tires order the wheels with the correct backspacing so you don't need a spacer. If you are using stock rims and just putting wider tires on them you generally won't need a spacer because you can't safely go wide enough to worry about it.
Tell us what you are doing or planning to do and we can give you better advice...
If I remember right, N.H.R.A. wants either the width diameter of the lug (7/16 in. lug = 7/16 pertruding from the lug nut) or 1/2 inch pertruding, I can't remember which but even if you're not planning on racing the car it sounds like a good idea.