How to decide who to tip?

Got to thinking about this concerning oldscratch’s pit thread about tipping bartenders. How is it determined what individuals providing what services get tipped?

I’m aware of recent threads such as this
http://boards.straightdope.com/sdmb/showthread.php?threadid=32988
discussing how much you tip various persons.

Listed are tour guides, hair stylists, doormen, bellhops, taxi drivers, casino personnel, etc. But why, for example, would you slip a buck to the doorman for calling you a cab, and not do the same for the desk clerk who checks you in? Or why tip the hatcheck person, the waitperson, and the bartender, but not the cook? Cab driver but not bus driver? Why not tip your doctor? Or a tradesman? Policeman or fireman? (Yes, I’m aware of legal prohibitions, but they certainly are providing a valuable personal service.)

I’m not trying to be snotty. I’m genuinely curious why has it been agreed that certain occupations merit tips, and others don’t? Does it rest on an assumption that certain jobs receive inadequate salaries? Or is it an attempt to receive better than average service?

A milder version of the tip is the “gift”. Have those of you with kids noticed the explosion of gifting occasions recently? Not only is it expected that you will contribute to a gift for the teacher, but also for the student teacher, the soccer coach, the scout leader, etc., etc. It kinda bugs me to keep getting nickel and dimed, and to be pressured to give a gift in certain situations.