Slashfood’s Hanna Raskin recently wrote in her blog that restaurant customers are doing themselves and their servers a disservice by leaving a “standard” 15-percent tip for good service.  According to Raskin (blogger, server and food-tour organizer) leaving a 15-percent tip isn’t just rude, it’s also unrealistic, insensitive, ungrateful and outdated. Think that $3 tip on a $20 bill is fine? Think again, she says. If you want to let your server know he or she did a fine job, you’re going to have to do better.   It seems that most of Americans agree with Raskin, as the national average tip rose to 19 percent in 2008.  However, I think tipping is more complex than a flat percentage.  For example, many college students don’t have much spare cash, so a 15 percent tip could be really generous and show that they appreciated the good service.  On the flip side if a well-off couple dines and tips 15 percent, it could mean they didn’t approve of the service.  All in all, a 15 percent tip is a 15 percent tip. And now that 15 percent is no longer the national average…