pluggy:
If you only want 1 per room, I'd find the specs for the LEDs (foward voltage, recommended safe current) and feed it with 12volts through a series resistor of suitable value. LEDs aren't rated by voltage, using a suitable resistor you could run them off 200 volts if you chose to (not recommended). Personally I'd wire 3 in series with 12 volts and a smaller resistor (it would use the same amount of electricity for 3* the light . Buying small (5mm) high brightness LEDs is cheap in comparison to the effort involved in positioning and wiring them. They can be had for pennies apiece on ebay. Running 1 LED from 12V is no more wasteful than using a 7805 to step down the voltage.
Hi Pluggy
So you're saying it may use less ( or very similar ) power to have, say 5 LEDs 2v 20mA in series with a single 100 ohm resistor connected to a 12v supply, than to use a 7805 to drop the 12v to 5v and connect a single LED with a 150 ohm resistor connected to the 5v ?