From a Sep 2006 article in The Washington Post on tipping:
"It's a common belief that tips are based on service but Lynn [
an associate professor at Cornell hospitality] argues it's all wrong. Research has shown that tips are rarely based on the quality of service, and are usually determined arbitrarily.
"Servers think there's a relationship between tips and service, but there isn't. How sunny it is outside literally has as big of an impact on a tip as the service," Lynn says.
One study showed that when waitresses wore flowers in their hair they earned 17 percent more than when they didn't. Another study suggests that waiters who squat beside tables receive an average tip of 18 percent, versus a 15 percent tip received by waiters who stand next to their tables.
Some waiters concede that tipping behavior has very little to do with service. Theresa Burkhart, a 31 year-old waitress and actress in Manhattan, says overtippers tend to overtip regardless of service, and undertippers tend to undertip; Canadians and Brits often tip 10 percent, while Japanese customers tip around 20 percent."
http://www.washingtonpost.com/wp-dyn/content/article/2006/09/12/AR2006091200809_pf.html
I do think tipping on a percentage is fairly silly. Flat service fees that restaurants are free to set and that you know in advance or are listed on the menu seems better to me. It'll never happen and I'll continue generally overtipping based on percentage, but it would be nice.
And I recognize that there seems to be a contradiction in
1) not balking at tipping on the full amount on food even if just a small number of (not overwhelmingly hard to prepare) very expensive items are involved
AND
2) balking at tipping full amount on multiple bottles of expensive wine
For me personally, it's the money involved in 2) that makes me balk. And if pushed, I would have to say 1) doesn't make a whole lot of sense either. Maybe I'm just used to 1) at this point or can more easily afford it so I've adopted it more unquestioningly.