I've actually worked as a server before (at a small, local restaurant), so I definitely agree that tips are appreciated. I also realize that my experience at a local cafe/restaurant that paid me well is different than those who have served at big chain restaurants. Anyway, I always give tips, but it is more satisfying when I feel that the tips were well-earned and also when I feel that tipping was a voluntary gesture of kindness on my part. (I'm still a young adult

so right now I'm sticking with 10-15-20, but I know that when I'm older and when I make more, this amount will go up more).
Also, as a student I've worked a slew of other part-time jobs, often working more than 2 or 3 at the same time in order to make ends meet. When I used to do the budget, I always factored in my wages for these jobs, such as serving. However, I did not factor in tips. Tips were something extra that went in my wallet (for my use or for donating), and I made sure that based on my working hours, my wages were enough to make ends meet (including some personal spending in the budget).
I guess this is why I dislike arguments from other servers that go along the lines of "Tips are important because if not, we can't make ends meet and etc, etc, etc. everyone should be required to tip b/c if not, my life is harder" I just feel that from the get-go, they shouldn't be relying on my tips to make their ends meet, because that puts a lot of pressure on me ! And I'M just trying to make my ends meet, too! Their budget should have been made flexibly with tips considered extra and it's selfish to judge people who eat at restaurants that don't give as much as expected. And it's this idea that there is an obligation imposed on you, rather than something done voluntarily for the benefit of someone (as social etiquette asks us). It's the difference between commanding (with emotional arguments) versus asking someone to comply.
I also don't like people who say things like "do you know how demanding this job is" as a reason for tipping, because when I was working, I went through the same awful conditions. I sucked it up and worked hard and did what I have to. I didn't feel resentment towards customers that didn't tip, and I worked without expecting tips (this makes life so much more easier). My coworkers probably thought I was cold though, cause I never related or joined in their ranting of bad tipping customers. I've also seen servers demand tips from customers (& complain about small tips), but won't tip their cab-driver or the grocer that helped carry their groceries (seems selfish to me). In general, I guess I really don't like emotionally charged arguments; as soon as I sniff any scent of something negatively emotionally charged, I instinctively don't like it. I prefer cold-facts such as "I'm a server - this is how much I make, and these are the conditions of my circumstances" or positive statements such as "you are making a difference/tips are appreciated." It's just the negatively emotionally framed ones that drive me crazy for some reason.

And overall, I think our society is being stupid in terms of how this system is set up.