tm,
Having instructed on both military 7 tonne helicopters and light aircraft, including close formation, I can confirm that the rotorwash definitely goes downwards; so aim to stay above it and you will be OK. If you ever see military helicopters in close formation, you will see the following aircraft "stepped up" from the ones in front, for this reason.
On the other hand, RAF fixed wing formations usually "step down" from the ones in front.
To put this in perspective, our regulations (RAF) allowed a minimum of one rotorspan between similar helicopters in formation.
Do beware the danger from downwash on your flying controls on the ground - hang onto them tight, if operating near taxying helicopters, even though your aircraft isn't flying! If you don't, the rotor downwash may blow your control surfaces to full deflection and this will possibly cause untold damage to your aircraft.