Edit2:
Reasoning:
Multiples of modded gridseeds run at what ever.
Example:
508*6 = 3048.
3048/10 = 304.4
508/10= some where around 50 but I found less hardware errors at a higher rate of 64-67.
Also, ppl use CPU's that use diff of like 4.
Even a single 350 kH/s Gridseed will submit one share per 24 seconds on average with difficulty 128. This is more than enough. Hardware errors shouldn't be dependent on difficulty and rather on frequency which you setup at your miners.
As for CPU: nobody is CPU-mining scrypt any more except botnets.