When you go to a restaurant, do typically tip?

Yes, of course.

Do you tip a set amount or do you vary based on service quality?

I probably average around 20 percent on average to decent service. I tip more around 30 percent on better service. The waitresses and waiters at the restaurants I frequent know I tip well too and thus I often find myself being offered more chips and salsa, frequent refills, and so on. I have noticed even more meat, rice, potatoes, etc. in my main meal.

Why do you tip or choose not to?

First, I am not a mean person so even when I get bad service (which is rare) I am not a dick. I don't leave a single dollar or 2 cents or something stupid like that. I simply tip 10 or 15 percent and leave. I may tell a manager that I received bad service and will consider not coming back but I don't pull shenanigans.
Secondly, I tip as I said around 20 percent on a typical bill. I mean if go out and get some Mexican and I have a 13 or 14 dollar bill I'll leave 3 bucks, maybe 4 if I get excellent service (again see above for what I mean by excellent). I am a little more conscious about the math but still not necessarily strict if the meal is something like a date where the bill is 50 or 60 bucks. Then the gap between average service and excellent service might be larger.

Pizza delivery, tip the driver?

Of course.

Why or why not?

For many of the reasons you stated. And, anybody that lectures you about why they won't tip you is an ass. If you had already given them their change I would just turn around and walk away as soon as they started in.

Other ideas in the area?

As far as tipping? First, I always try to tip cash but I admit it is harder nowadays because I use my debit card so much. Oh! I do hate it when I give a server a 20 dollar bill and my bill was something like 13 and some change and they bring me back a single, a five dollar bill, and some loose change. Do they think I am really going to leave the five for a tip? I have never been mean enough to do that but to me that is bullshit and by all rights I should leave them the dollar and the change to teach them a lesson. Instead I usually tell them to break the five and hope they realize their gaffe.
Second, I tip take out if I know the restaurant and like the people. If it's just take out for me and the bill is say 10 bucks then I'll usually just toss a buck in their jar or write a buck tip on the receipt. If it is a bigger order then I'll leave a few bucks but I don't think it ever comes out to 15 or 20 percent.