Exactly, don't expose yourself to licensing grief. Once the computers are out of your hands the recipients can do what they like with them, including pirating Windows 7 or XP if that's what they want. But that will be Not Your Problem.
Free Geek in Vancouver, BC does this. I don't know if they've ever done a survey where they follow up 6 months later and see what OS the computer is running now. Would be interesting.