I realize this is an old post but thought this may be helpful to others trying to use URC Dimmers (600M/6MLV) for LED Lighting.
I could not find the minimum load requirement for these dimmers and after talking to Lutron they also had no documentation on the minimum load requirements, but going off their RadioRa line said it is probably 50 watts minimum load. The higher the load (watts / number of bulbs) of the LED lighting you are trying to control, the better your dimming results will be.

Using a Lutron LUT-MLC (resistor) will let the led bulbs turn completely off and help with dimming a little (got me to about 50% before flickering the bulbs). Others have also suggested using a 1.5 kOhms ±1% 25W Wirewound Chassis Mount Resistor (RH0251K500FC02 / KAL25FB1K50 )

Using a Lutron LUT-LBX Synthetic Minimum Load For use with Reverse and Forward Phase Dimmers will let the led bulbs turn completely off and help with dimming better than the LUT-MLC. The LUT-LBX got my LED bulbs dimming down to about 15% with no flickering. EBay is your cheapest bet for the LUT-LBX. The LUT-LBX requires a double gang box for in-wall or surface mounting, but can be added anywhere after the dimmer switch and the first lamp.