I would say the same principals apply as with every large plane. Stay behind and above their flightpath.
But I don't think it's as bad because it probably leaves just a bunch of jumbled up air instead of a huge clean rolling wake.....
When a helicopter is in forward flight, the air gets processed through the rotor fairly quickly and the vortex ring that builds up in a hover is broken up and pushed back behind the machine and probably gets "processed" again by the second rotor....
However, whatever you do: Don't get underneath a Chinook ! ! !