I drive it first and see what it needs, come to the conclusion that if it needs to be faster I have plenty of other cars for that, then put some stickier/less sticky tyres on.
If I have a target in mind for a particular car (a certain race, make it a drift car or to make it handle like it's on rails) then I'll achieve it, but if I buy it for no reason other than because I want it, then I'll probably leave it stock. Also I do buy cars just to drive them stock so I do that a lot, too, though I don't count a different tyre type as a modification. I refuse to believe things like the Enzo handle like that fresh from the showroom...