My parents always told me that if you intend to tip less or not at all then you have an issue worthy of management, if it's not worthy of management, then proceed normally even if not completely thrilled with the staff. Their logic being the tip is part of their pay, if you're docking their pay, it must be severe and management should be involved.

So, I've tipped 20 percent to mediocre waitstaff plenty.

I also have this bleeding heart issue where I feel like maybe I was there on their bad day and I tip well because I've had those days :)