Depends on your launch profile, of course. You could try to stay air-breathing as long as possible while building some of your orbital velocity. I'm not sayin

Depends on your launch profile, of course. You could try to stay air-breathing as long as possible while building some of your orbital velocity. I'm not saying there's One True Mission Profile here. Depending on your needs and constraints, it might be a good plan in some cases. See also: https://xkcd.com/1244/ https://xkcd.com/1244/ (sometimes)