I left my plastic rivets off - the bumper stays on just fine. Plus, it's likely that I'll take the bumper off again, and I'd rather not deal with the rivets. Without a doubt, removing those cost me the most time (in hindsight, I'd just clip them off with wire cutters).
Dabitz - if you even have an inkling of mechanical ability, you can do this yourself. Or, drive your WK2 to Northern VA and I'll do it for you.