As someone who worked their way through college delivering pizzas in a very busy suburb at all hours, to all kinds of people. And as someone who has seen her friends work 2 jobs and come home to their child that runs up to them to take all of the "tips' out of their pocket and puts it into their vacation or b-day bank I am always someone who tips well if the service earned it. Nothing would ruffle my feathers more when I was delivering pizzas to a $300 plus order from a business, where I drove in my own vehicle during lunch rush hour in 100 plus degrees, toted 15 large pizzas up a flight of stairs, returned for their 20 salads and then again for the eight 2 litre bottles of soda to find a company check for the EXACT amount. I made $2 less than minimum wage, earned $1 on every delivery and the rest was tips. This paid for wear and tear on my vehicle and gas. It's amazing how many people I encountered who never thought that tipping a pizza delivery driver was part of the job. Assuming your pizza doesn't show up an hour late, the driver drunk and your pizza on the top of the box or half eaten. Also important to mention I got to know the non tippers and I would go out of my way, run an errand or take every other pizza to where it needed to go before I even entertained the idea of stopping at their house.
The general list of professions I do tip and always have are servers, housekeeping, hairstylists, delivery drivers (adjusted amount depends on what the company charges me upfront for the delivery), dog groomer, taxi drivers, bell hops, valets, the person who takes your bags at the airport or hotel, and yes even the sonic car hop (change usually). Professions I do not regularly tip are my dr (because please), mechanic, home repair person, any person I have prepaid a delivery charge to or that uses a company vehicle to deliver my product, and government workers.

I have been known from time to time depending on how often I go to my veterinarian or doctor to bring cookies or a treat for the staff if I have been a regular for whatever reason and the service has been outstanding.
My general rule of thumb is if you are providing me a service simply because I do not want to or am too lazy to do myself then I am going to tip you for doing something I probably could myself. No this doesn't apply to my hairstylist, but again she has a pretty important job and I like to keep her happy. I could eat at home and prepare a meal myself, if I don't want to I will tip you for helping me out with that. I could groom my dog myself but she'd probably run away from home.

I could park my car, take my own bags to my room, go get my own pizza, etc... so this is the standard I use. My normal scale is 15-20% and if the service is awful I am not afraid in the least to toss a penny on the table.
As for the tip jars in my local BBQ joint and drive through coffee shop, I too look at them and think about adding one to my own workspace.
