A heel has to earn your boos imo. If you like them but boo them anyway they're failing to do their job. I think we're past the point in getting people to sheepishly boo the heels and cheer the faces, but that said Cena was so hated in 2007, even with a heel turn, he still would have been booed. Once he started getting cheers, you turn him face again. That's how it used to work. Before 2001, WWE would have never let their top baby get booed every week.