Actually, for me there's only a tiny difference between a high memory clock and 300mhz (though with the higher clock, I get like 2 or 3 extra mh/s). But it would be better to keep at at around 300mhz, as others have said, it lowers your temps slightly and might even increase your mhash!
That is only for 5870s and 5xxx series mostly. I think the 6xxx series can only go down to 900 mhz on the memory.