it would need to be pretty negative offset, or low backspace. here is what a 15x8 with 3.5 inches of backspace looks like without a spacer.
IMG_3088 by
Joe Doh, on Flickr
later I did 15x7 with 3.5 backspace on the same truck, still no spacer.
IMG_0881_zpsv8or8q9w by
Joe Doh, on Flickr
I am custom ordering wheels for the 49 I am working on now, so I will run no spacers on that one too.