The easy answer is that you shouldn't mix the batches. But really, we still don't have sufficient information to know.
Do you currently record materials against batches through to final product (full traceability)? This is expensive, the customer had better be requesting and paying for this service. If you're providing this traceability for free the company is getting ripped off.
If this part was being bolted onto the a space shuttle, would each part have to be traceable to its material batch? Yes.
In your case, if all the batches of the same part have material certificates that all say they are within spec, what's the big deal?
How reliable at material test certificates anyway (not much from my experiences)?
Is each batch inspected for visible compliance to the correct ID CODE for the material? Did the guy marking it at the foundry mark it correctly? What about half lengths that may not be marked?
What happens to the different material batches through you processes before they get to the grinder? Any chance of mixing?
What does the customer stipulate?
What does the Work Order state?
If the part was being used in a garden shed does anyone care about the material?
Sorry, I've ranted again.
------------------