I have noted that some people are using different wheel spacer thicknesses, front to back. 25mm/15mm.
Does the OEM suspension provide different front/back wheel tracking?
Is there a reason for this asymmetric spacing, aside from somebody's cosmetic preferences?

 

It's probably a combination of cosmetic preferences, and wheel dimensions such as offset, backspacing, etc. Not every OE wheel had the same offset, backspacing, and width. Larger wheels (19" and especially the 20" wheels) had a slightly less tucked in stance.