Exactly what Jerry said. Why wouldn't you drop start a saw. It's easy fast and efficient and I would say no more dangerous than the 'log start' that Darin described in another thread as long as the chain brake is applied.
Again for me it's the only way I ever start a saw.
OK, I'll chime in on why we shouldn't drop start saws.
Not to claim I never do, but I try to avoid it. Not for reasons of safety, re loosing control of the saw...as Jerry rightly says, setting the chain brake takes care of that.
Drop starting a saw is hard on your body, especially over the years. You essentially are throwing the saw away from you with one arm and pulling the starter handle towards you with the other. This torques the joints of your shoulder, elbow, and wrist in an unhappy manner, and over time many will find that those joints don't like it.
Same is true with the "bar over a log" method, but to a lesser degree. I only use this method with larger saws sporting longer bars, generally when they are warm, not cold starting, and with comp releases.
Drop starting smaller saws is actually harder on your joints than larger ones, because there is less mass and inertia in the lightweight powerhead, so the torqueing is more extreme.
In the tree is the most challenging situation...I like Frans' post...do what works best, safest, in each particular position you find yourself in. If you can brace it some way, it's better for your body in the long run. Drop start it when you need to, but look for alternatives.