When I worked as a tech, I didn't have the spare time to read your "stuff"
If you are having a software problem, then the shop would need to sort that out. That will be difficult, if not impossible, if you don't give them permission to do that - which would include relevant passwords.
I HAVE had customers watch while I worked, because of the nature of the files that were there (private medical files, or business-critical software) - but they asked if that was possible. As I recall, the shop charged it as an onsite job, rather than a normal in-shop checkout.
I know that some folks here will do a backup, then erase and install a fresh system. If it is quite important to you, then you could also have a replacement hard drive, with a fresh system install, and you could swap that into your MBPr before you take it in.
Of course, if your problem is related to the software that you have (along with your important files) then that fresh system may not help you at all.
If it's just hardware, then the shop might get by without using your boot system at all. If you need to make other arrangements, call and ask them.
I suspect that you can also ask at the time that you drop off your computer for repair.
If you don't get an answer that you feel comfortable with, then don't leave it (unless you actually do want it repaired.)