I believe flights would usually gain altitude before crossing the lines. Low and you are a sitting duck for both MG and aircraft.
Balloons are worth climbing for, as you can dive on which helps you avoid the above dangers more. But as Craighorn says simply press next waypoint (actually Shift+W) to skip to the next waypoint (once the flight is formed up) and off you go no climbing first.
I get reaching altitude before reaching the lines, but as I said when it's a twenty to thirty minute flight
to the lines it seems silly to me to spend another twenty to thirty climbing before you even start heading for them. Even if standard procedure was to do so I'm guessing that at least some pilots on each side would have come to the same conclusion, making the Shift+W solution not nonhistoric, imo. Thanx for the info Creaghorn and Pol.
