Bomber,
It helps...but it has to be aggressive....a change in speed, height, and and flight path makes you a harder target....not safe. Different weapons have different capabilities. A .50 caliber sniper rifle at moderate ranges on an aircraft landing or taking off with no deflection shots is deadly....an AK-47 on full auto at short ranges can also be deadly....RPG's are real helicopter killers....no small wounds with those things. MANPADS at altitude....depends upon which type...can be lethal beyond belief.
Each threat presents different requirements to defeat....each can be....but when threats are combined into an "integrated" threat scenario....one finds it impossible to be completely safe. Fly low to avoid MANPADS and you enter the envelope of small arms fire. A .51 Caliber machinegun will eat your lunch right on up to 5800 feet or so.