I'm new here and wondering the same thing. I've read that editing and tweaking are encouraged. And I've seen increases most of the time for fixing mistakes, etc.
On the other hand, a few times when I've felt I increased quality, I've had drops in score that haven't recovered yet.
1. My favorite hub dropped after editing from a score over ninety and has never come back from about 86. But it did receive an Editor's Choice award, which I'm happy about, as I'm happy with the piece (http://hubpages.com/hub/Wakiki-Family-Travel-Tips) though I may have decreased quality by over-thinking editing, having been within my first few days.
2. I made what I thought were improvements to two hubs yesterday that I had previously improved upon. Yesterday's edits dropped them both to 76 (almost a ten point drop for http://rockymountainmom.hubpages.com/hub/Ten-Compe... The other that dropped a lot and suprised me was http://rockymountainmom.hubpages.com/hub/Travel-Mi...
I'll be focused on quality over points, and am trying not to over-think it too much. But in at least most of these cases, it's been useful feedback that has pointed me to needed changes. My most recent hub was published with room for improvement and increased in points when I made my initial needed adjustments.
This is leading to the type of practice I was hoping this site would provide, so I'm a bit confused but overall quite pleased.