It's a hard one to call, though I'd agree with Phil about the fire risk.
I don't see why a laser printer should use more toner on start-up (unless it's set to automatically print a test page), it's not like inkjets which often go through a head-cleaning exercise.
Power- it will use less power if it's switched off, but the surges and thermal variations at switch-on are likely to accelerate its demise (though electronics are a lot better than they used to be in this regard). So you have to balance frequency of replacement with the (low) power consumption in sleep mode.
I leave mine on, but I don't think I could give a cast-iron justification for either scheme.
Which isn't what you wanted to hear, sorry about that