From my experience a 20 offset on the rear only works well.

If you ad them to the front it will bring the tire out of the fender enough to throw road debris all down the side and create nicks in side mirrors.

If you get the front to wide it creates an improper hub bearing load and could wear these out prematurely.

Guess it comes to personal taste and opinion. You won't have an issue selling the extra spacers if you do not like.