I hate seeing the auto-gratuity at the bottom of receipts, but I've also never left the tip portion blank. I even grumble about the auto-gratuity when dining as a group. The tip should be based on the server's work, not simply because you were served!

My standard for tipping is usually between 20-25%, but if an auto-gratuity has been added, I don't add anything extra, even if the auto-gratuity is only 15% or 18%.