Replacing stem bolts Rather than replace the stem on every rusty bike I come across, I want to replace some stem bolts which are rusted, cheap original ones. I know they can differ in size a bit so I would get some that fit nicely the profile of the existing stem. 

Are there any specific requirements for bolts to do this job? Can I just bulk buy some stainless steel socket head cap screws?

In particular this is for other people's bikes, or refurbishing ones to be sold. I would maybe try things out on my own bike but don't want to take any risk with someone else's.

Most jobs on the bike I would be more confident about subbing bolts in, or they usually have better lasting bolts, but stem bolts seem really safety critical to me but often rust and deserve replacing. Am I worrying where there is no need?
Most bolts should have their tensile range marked on them somewhere, often stamped inside the hex cap. There should be a short number - 3.6, 8.8 etc.. Even a fastening rated as low as 3.6 is capable of 27 tons per square inch, more than 300n per square meter. A stainless steel M6 can probably handle 1.25 tonnes, the aluminium you're bolting into / through will fail before that.