Actually, The factory offset is listed at +48mm although if you look on the inside of the rim spokes it does say 50mm. The 6 mm difference between the stock 48 and the 42's your looking at is about 1/4 inch difference (closer to the fender). I suggest you go and measure the clearance on your stock wheels at full lock left and right and see how much clearance there is.
Don't forget we're talking about positive offset. The only time you'll need wheel spacers is if you find wheels with MORE positive offset than stock, say a 58mm vs 48mm.
There is a lot of info on this previuos thread: http://www.forteforums.com/forums/koup-wheels-tires/818-help-wheel-fitment.html
Cheers.