Do you think there are people who dont tip at all? My friend and I were talking about this because she used to work in a restaurant. She said every now and then people would not leave a tip. Do you think they forgot? were angry at something yet never spoke up? or just dont feel the need to tip?
I think occasionally someone forgets to tip, heck I even had a customer (a local pastor) forget to pay his bill once.

He was chatting with another customer and just forgot. It happens.
I worked 12 years as a waitress. Most people tipped, and most tipped well. There were a few who never tipped. Ever. I didn't understand how they could come in week after week and never tip any of us. And truthfully when I knew they were not going to tip, I didn't go "above and beyond" for them like I did with other customers. They just weren't worth my time. If they needed drink refills and a table right next to them (who I knew tipped) needed them, who do you think was going to get their refills first? I'm not saying I didn't take care of the people who didn't tip, I did take care of them and they did get drink refills, or whatever, but just not as quickly as others.
As a server I would hate to see tipping go away. I made good money working 15-20 hours a week as a server. I would not have made nearly as much if it were a minimum wage paid position. And as a customer I don't mind the tipping policy at all. If tipping went away then restaurant owners would have to pay their servers a higher wage and they would just raise the prices accordingly on the menu. We tip well, as I know serving can be a difficult job. Most times we tip 20% or more, if service was just okay they'll get 15%. If it was obvious they preferred standing around chatting with other servers and we were neglected, or if they were rude I have no problem in not leaving a tip (or a little change by the plate, so they know we didn't just "forget").
I think tipping is not really "optional" since the server generally makes $2-$3 an hour and is expected to make up the difference in tips, and I'm in the camp that thinks if you can't afford the tip don't go to a sit-down restaurant to eat, McD's is right down the road. But I do think it is "optional" in that the customer should decide how much the server is worth, whether or not they did their job well, took good care of you, were attentive but not intrusive, etc.
As for reporting our tips to the IRS, we did have to write down our tips each night when I first started there but after awhile we were just taxed on 8% of our sales and no longer had to report our tips.