I think in theory the spacer with bolts attached are when they are much wider ie 30mm+ as it gets to the point where the wheel nuts themselves would have to be tooooooooo long ;-)
Personally I would never go wider than 30mm anyway and on most cars you should not need to unless you have got the wrong spec offset in the first place.
The important thing is that they are PCD 120 (so the holes line up) and the correct number of holes (but most are generic and have more holes than you need). anything over 5mm really should be hubcentric ie have a little flange that side in the hub to help secure it in place. You don't really need this with 5mm or less.
here is decent video about them:
http://www.youtube.com/watch?v=1gU_XcLxcPw
People tend to forget to factor in the cost of longer bolts and security bolts which can add c£70 to the cost ie double it !
ps TPI are a great make. Branded quality but at a cheaper price and they look cool as well. They do kits with longer bolts included.