From HN (https://news.ycombinator.com/item?id=34394351):
If you assign a block of time to quality, you risk people taking that as an excuse to not focus on quality outside that block.
I feel like this has been my worry and experience for a while. Good teams I have been at shipped quality by default. They would not require bug blitz or similar. Except for well defined area of complexity or integration with another team.