Try the LEDs first then see if they work OK. Add the resistors if you have to.
21 watts = 1.75 amps = (roughly) 7 Ohms per corner. 8 ohms will be fine though, BUT make sure they can handle the power (21 Watts, remember) so your common or garden 0.5 Watt resistor from Maplins will pop in no time.
Also bear in mind the heat that will be dissipated by the resistors so keep 'em well clear of plastic parts. I'd recommend heavy duty resistors like these...