Ask dentist LOR before or after start working for them?

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I would ask after the fact. If you didn't know the dentist beforehand it's kind of tough asking them straight out of the gate. Most dentists probably know students will eventually ask for one anyway, I would just build a rapport with them first. Show up on time, work hard, ask a ton of questions, etc.
 
Agree with above. If you want you can preface it in casual conversation ("at some point I may ask you for a letter of rec if that's all right.."), but there's no need to emphasize it too much until you've worked for long enough.