Use where would you use when you want someone to picture a possible situation, not report a habit. You might ask a colleague, Where would you use this kind of email opening? For their actual routine, ask where do you use it.
Situations and examples
Picture a case that hasn't happened yet, and where would you use fits. You're helping a friend choose a tool and ask, Where would you use this wrench? The wording invites them to imagine a context with you.
The would marks the situation as imagined. You're drafting an email and wonder about a sign-off, so you ask a colleague, Where would you use this kind of opening? In this phrase, that conditional wording points to a possible case rather than a report.
Planning works the same way. You're packing for a trip and ask, Where would you use this adapter? The trip hasn't happened, so the imagined-case reading fits what you're picturing.
For a habit, switch to the simple form. Where do you use this app? asks about their actual routine, while Where would you use it? explores a possible case together.
If the situation already happened, the conditional sounds off. Your friend just showed you an edited photo, so ask, Where did you use that filter? It's a real event, so the simple past communicates more directly.
Keep where would you use for the imagining case. Otherwise the simpler form is the more direct choice.
When the simpler form is better
Stick with where do you use it when you're asking about someone's routine. A colleague always signs emails a certain way, so you ask, Where do you use that sign-off? You want their habit, not a picture of a possible case.
If the situation has already happened, the conditional sounds off. Your friend just showed you a photo they edited, so Where did you use that filter? fits the real event better.
When you're giving an instruction rather than exploring options, the simple form works too. You're showing a new hire the template folder, so ask, Where do you save client files? There's one established answer, and would adds a hesitation the moment doesn't need.
Save where would you use for the imagining case only. If you're not asking someone to picture a fresh situation, the simpler form communicates more directly.