Errrm, what do you mean, like when you buy the rims new you'd have to buy new tyres as well?
No, if you have lets say, an 18" wheel already with 80% thread left, and you'd now like to change rims, you can only buy the rims and still use the tyres with 80% thread left, a simple take off and put on jobbie.
But if your 18" have about 30% thread left like mine does...you'll have to replace boss.
Hope that helps