Armotw - It all depends on what angles you are trying to shoot of the subject aircraft. I have found that if the photo ship is flying around the subject ship and they are working in front of it, then the pilot has a tendency to always pull a little back pressure on the cyclic when looking over his should to see where the other aircraft is. Invariably that brings the airspeed back and there have been plenty of times when I have wondered why the subject ship is slowing down and we, the photo ship, have come back to about 10 knots.
Therefore if I have the subject ship any further back than either 9 o'clock or 3 o'clock, depending on what side of the photo ship I am shooting from, then it is way way easier to have them formate on us. The other day on the Childflight shoot we had the BK-117 camera ship going sideways down the beach at about 30 knots with the SA-365N2 facing the doorway doing the same speed, and the shots were staggering. (Drop me an email and will send you some of the samples if you like). This was the best way to get a great head on shot.
Also getting a near vertical shot from straight overhead was easy to do as with the two pilots in the aircraft, the left one took over since at that time I was sitting in the left doorway. He was very very experienced and got me the straight above one I was after. Hope this gives a little insight.
Heli Ops.