A while back I left my car outside overnight (i.e. not parked in the garage as usual). When I hopped in my car to drive to work the next morning, I noticed a blob of bird poop had splattered on my driver side window. Fortunately it didn't block my view of the rear view mirror, or I might have needed to take care of it immediately. But it posed no immediate hazard.
So I left it there for a while. I let it serve as a reminder of a simple lesson for me: If it's crap-let it go!