+22 is the "correct" fit.
I've read a lot that +12 will be fine on a 18x9.5 rim, BUT there are many suspension factors that affect fit. On a stock lowered car +12 might be fine. But when you start playing with camber and caster and aftermarket suspension arms, it all goes out the window.
I have 18x9 +30 R34 GTR rims, and if I add the quarter inch plus 18mm I get the outside of the rim sitting 24mm further out. On the rears of my 32 GTR, given my suspension settings (and other factors such as 3piece rotors with a thicker alloy hat pushing out the hubs marginally), this will protrude the wheel outside the guard. Now I could tip some more -ve camber but it is set up to handle, not to fit a set of wheels in. So unless I want to compromise the suspension set up, +12s will be too wide.
On the front, the +12s will fit fine in terms of not protruding from the guard. However, as I have a stack of caster, the front wheels are further forward in the wheel arches than stock. Therefore as you turn the wheels, the outside edge of the tyre/wheel catches the front of the inner guard. If you increase the offset, this will mean the wheel will contact the inner guard with even less lock applied. So again, I don't want to change the suspension geometry to fit wheels, I'd rather get the right fit wheels to work with the suspension set up that is currently set up for handling.
hope that helps.