If your worried about how you're subordinates are addressing you in an email you need to get a life. Not sure I'd want to work somewhere where my manager is stuck up / has an inflated sense of his importance. Maybe I've always been lucky to have relaxed managers, sort that I can go get drunk with.
Overall I'd say I hey'd about 80% of bosses and 95% of employees at least some of the time.
Depending on your answer I may have been backhandedly offending managers for the past 15 years.
I use it frequently with multiple managers and directors. It entirely depends on your relationship with the person. Sometimes you can cultivate a relationship where being so formal and subservient is unnecessary, if not counter productive.
Edit: Not in California, or the US.
I've actually never worked anywhere such formality was expected, and suggesting something like that would probably get you some weird looks. I wonder if it's an industry thing? Perhaps some industries tend to enforce politeness/formality between rungs in the career ladder more than others?