I have 16x8's on my Yukon XL now (profile pic), -6 offset as opposed to the "stock" +25. So, basically, they stick out just about 1 1/4" more than stock, just inside the lip of the fender flares. I have 1 1/2" spacers on the rear only for right now.
Previously, I had some 20x8.5's, factory offsets - I had 1 1/2" spacers on the front, and 2 1/2" on the rear. Sold the 20's, went back to the same 16's, WITH the spacers.... tires now stuck out about 1 1/2" in the front, and 2 1/2" in the rear.
Have a set of GMC Sierra 20x8.5's I recently picked up, will be running either 35x12.5's or 295/65's, and will be putting the spacers on both front and rear.