It is discretionary unless you have large parties. Sometimes when you have a huge party they apply the gratuity at the top. The downside is if you leave a crap rip you risk getting shamed by the scorned server on social media. I try to tip big as a rule simply because I know how low servers base pay can be and many times they need to pool their tips. It's a job I certainty wouldn't want to do. Looking back, my five dollar tip for that pizza was probably a bit excessive given the already applied delivery fee. Thanks for the awesome comment and the upvote!