Bitlocker? FileVault? If you're cracking those, why the fuck are you working at a Best Buy?
Shitty people will do shitty things. That said, if you don't give your password, be prepared to have the technician test all sorts of stuff in front of you. The selfie camera, ear speaker, microphone, etc. sometimes are mounted on the screen. If there are problems, the tech will need to redo the repair. Not advocating for giving your pw, but be prepared for the process to be less convenient.
Edit: My bad, should have clarified I'm talking about phones exclusively. If you're worried about your computer, create a non-admin user and give them that password. If they had the skills to bypass that, they wouldn't be working at a repair shop.
I'm just happy you got to use the new term you learned! Color me impressed!