Crystal clocks (quartz and the like) use an oscillator where the frequency is determined by the size and shape of the crystal itself, not voltage or current. Likewise, the venerable 555 timer IC's oscillation frequency is based on the ratios of resistors and capacitors, and as long as the voltage supply is enough to get the thing working the frequency should stay constant. Another way of doing timing stuff would be a microcontroller of some sort, but those generally also have set clock frequencies that are pretty much entirely independent of current and voltage. And all of those components rely on DC, which unlike AC really shouldn't have a base frequency at all, as it's just a constant stream of electrons from negative to positive. (At least that's my understanding of how it all works; I've been getting my soldering iron wet with some photo-related circuit design lately and I've begun to both love and hate the 555 with equal passion.)

The thing I'd be worried about is a slight change in voltage screwing with the brightness of the enlarger bulb. Before I got a timer (and a brain I guess) my Omega was plugged into a power strip along with a little radiant heater thing, and any time the thermostat would switch the thing on my enlarger would dim for about a half-second. A power conditioner would take care of that problem, but my solution was to just put them on two different circuits.