it all depends on what tires you choose, 18 is pushing it unless you get low profile tires
and it also depends on the width of the wheel, the backspace and if it has room to clear the calipers(which most should clear stock calipers)
some of the biggest mistakes i've seen involved tires that were the same size but had differnt backspacing and/or not enough room for calipers. saw the aftermath of a guy who got new calipers that didn't fit under the stock rims....not pretty.
so, take careful measuerments, OR find somewhere or someone that has the exact limits that we can fit. you can never be too careful with stuff like that.