If you are doing buy and hold, why do you care? We are living in Asia zones, think about your family precious time, your personal health, your emotion involved... We have to strike for a balance.

if one is a "long-enough term" investor, one should not mind to sell on the second high. but this definitely doesn't apply to speculator.
so which one are you? you have to ask yourself

we, the every individual are the only ones that ultimately define all the rules for ourselves. if there are no rules, I would say it is dangerous. Even " buy and hold S&p500 for 10 years and would never sell, no matter what" is a very good reasonable rule". I think every one would have something similar to this at the point of investing(speculating)