Since I'm an instructor I have several transmitters that I use as buddy boxes. I have 2 futaba's, 2 airtronics, and 1 Hitec. Since they are only used for buddy boxes I have taken out the batteries and the crystals. There is no chance that they can interfer with the student's plane or any others in the air at that time. I know that eveybody can't have a dedicated buddy box, but for the most part you should at least take the battery out of the buddy box (with the exception of the JR that needs the battery). Because the instructor is working with both boxes there is little chance that the students buddy box will interfer with the main radio of the instructor, but it is very likely that it could interfer with someone else in the air at the same time. In my mind, it's always better to be safe than sorry.

Just my .02 worth