BROOKLYN - Many Brooklyn residents say they feel obliged to tip for services during the holiday season, although a new study says that more than 50 percent of Americans fail to tip during the year. According to Consumer Reports, 54 percent of all Americans say they did not tip at least one person in 2011. Etiquette Now, an online etiquette forum, offers tipping suggestions based on different job types. It says occasional workers, such as gardeners and barbers, deserve a one-session fee, while full-time employees, such as nannies, deserve one-week?s or two-weeks? pay.