gleaf raises a valid point, if you use an enlarging meter you can throw it off.
I personally took the safelight off the timer after several near misses where I was using the timer (a GraLab 300) to time development, and being horrified that the safelight came on when the time was up. I was developing sheet film in trays!
I had to rearrange things to open up an electrical outlet in the wall for it. So sometimes the "safelight" socket of a timer is the most convenient outlet.