DFC:
The comment about DME was fair but some of the rest is harsh and a bit suspect.
I didn't understand the bit about
: "Since most light aircraft pitot heats are powered by DC then even having the cable passing by the compass, there would be no change in effect by having the switch on or off." - one of us needs to re-read their electromagnetism books
I didn't know about the 5 degree deviation limit from electrical services but the deviation card WILL only be CORRECT when all the services are on. 5 degrees may or may not be significant based on what you're trying to achieve at the time. There's an old story:
Nav to pilot: turn 1 degree port
Pilot to Nav: I can't fly that accurately
Nav to Pilot: OK, turn 5 degrees to stbd
Pilot to Nav: steady
Nav to Pilot: turn 4 degrees to port.
The comment about permanent magnets in headsets is certainly true if they're very close to the compass but they're fairly well shielded and I seem to recall that the field decays as an inverse cube law (may be wrong), so at normal distances the effect would be negligible.
HFD