I haven't seen too many blog posts on this. When I go out to eat at a place where there is a gratuity involved, I consider a gratuity to part of my expression to the kingdom of God in someone's life, especially if they give me good service. Its part of my silent giving that I give to give glory to God and encourage hard work and service.
My working base is 20%. If you give me average or slightly below average service, I'll tip you 15%.