the stock wheels are +40 IIRC

the offset will depend on 3 things:
-what kind of brakes you have
-do you have coilovers?
-what width rim do you want? (and do you plan on perhaps using spacers?)

-brakes: some of the Z brakes are wider in the caliper area due to being dual piston. they come in 26 and 30mm varieties. offset makes sure the innerside of the spokes isnt rubbing against the caliper (caliper and/or inner spoke grinding (LIGHT removing of material) has been done on rare occasions, but is not recommended)

-coilovers: same reason. since coilovers (i.e. tein, jic, buddy club, zeal, veilside, cusco) are one piece, spring and shock assembly are together as one assembled part and the spring perch is usually lower or somehow sits where it can interfere with the inner lip of the wheel or tire and can rub on either.

-width: using a 7.5", 8", and 10" wide wheel all require different offsets IF you want to achieve the perfect look and no fitment interferences. this includes the tire size as well. IIRC, the wider the rim, the lower the offset has to be, but i dont think it applies all the time. fitting the tire wheel inside the fender is also a problem that can arise. sometimes rear fender lips have to be rolled (push the lip up so it doesnt grind into the tire when the car squats) or removed.

spacers are merely for getting offset w/o having the perfect offsetted wheel. (such as rare "jdm" wheels that came over to the US and you really want them but to get the right look and fit, a spacer can come in handy --- or if a wheel only comes in a maximum of 7.5", and you want them but have them a little more flushed looking with the car, spacers play a part here as well)


i will be using 16"x7.5" wheels later on when i finish motor/brake upgrades, because they are what i like and will be having them custom coated/colored, etc. they only come in 7.5" that i know of, but if i cannot find them wider, i will be using spacers to give the offset a little better look.

spacers sometimes require longer studs