Somethings I have noticed with helos in OFP and ArmA.
1. I find it's better for the transport helos NOT to be grouped together, as they seem to like to fight over the same invisible H
2. The above also happens when they are not grouped together, but you can stagger the waypoints of the individual helos so they don't go for the same H. Stagger the H's also.
3. Helos always go for the first invis H they come to that's vacant, even if their land waypoint is on one that is farther away.
4. A visible H takes priority over the invisible H's...they'll go way out of their way to alnd on it.
Most of these observations are from OFP, but they seem to be holding true in ArmA also.