Does a laser printer use toner when switched on/off
Inkjet printers charge/clean their print heads during start-up, cartridge/head changes and in intermediate head cleans during long print jobs and actually pump ink out the nozzles into the waste discharge tray (opening up defunct inkjets and examining this discharge tray will show that at least 1/3 of the ink run through the printer during its lifetime ends up in this discharge tray).
Fortunately, Laser printers work a little bit differently in one important way.
Monochrome Laser printers go through a start-up routine to check for jams and make sure the paper path is clear. While the parts rotate, toner is circulated through the mechanism that transfers it to the drum, but because the drum has no charge on it, no toner gets transferred to the drum and gets returned back into the cartridge to await an actual print job where it actually gets used.
EDIT: Color Laser Printers have a slightly different problem. They have to keep four colors in registration (synchronize where the colors land on the paper fed through the printer) and therefore do a calibration routine which will use toner in this process.
If you have an art department, and they still have the stuff, a piece of rubylith tape does a good job of dimming out those over-active LEDs.
The printer will not use any toner while in power save mode or during warm up. You can safely use either power save mode or just turn it off. If you aren't going to use it for long periods of time, OR the flashing light bothers you, I'd suggest turning it off.
A cartridge vendor alleges
Reduce Stop/Start cycles
If you are using a laser printer you may notice that the printer does some whirling before and after every printed document. If you are printing a single page the printer goes through a start-up cycle, prints the page, and then goes through the slow-down cycle. The additional time and turning incurred during the start-up and slow-down cycles can greatly wear out the components inside your toner cartridge causing to lighter printing and increases the likelihood of developing defects. While no printing is occurring, the components in your printer and cartridge spin just as if something was being printed. In addition, toner continues to accumulate on the drum even though there is no printing. This extra toner is swept into the waste-bin of the cartridge, reducing the supply of toner available for printing.
My emphasis.
A test of software that claims to reduce toner usage incidentally mentioned
The cartridge was weighed after printing 500 pages so that any cartridge start-up toner usage did not skew the results
The Wikipedia article says
Some color laser printers, notably some Lexmark models[citation needed] run "calibration" cycles even when no printing has occurred for weeks. These are widely reported[by whom?] to waste a significant amount of toner from each reservoir, in addition to consuming electricity. This has a significant impact on printing economy, especially in low-volume applications. On some models these calibration cycles can be disabled via a menu choice, for others the printer must be unplugged to avoid this waste. Printers that have this issue have a replaceable "waste toner bin", which is another periodic operating expense.
A Lexmark document "Understanding Print Cartridge Page Yields" says
Calibration
Many Lexmark laser printers and MFPs use a calibration procedure to improve print quality and maintain print consistency throughout the life of the device. Calibration cycles use a small amount of toner each time they occur. Factors such as a large number of power-on and power-off cycles, cartridge movement from device to device, and abrupt or frequent changes in office temperature can increase calibration frequency