Personally, I like to ask (I want to clarify here that by "asking" I mean giving your professor a heads up, your sincere request should come later) my professor for letters of recommendation like 4-6 weeks into the semester (you should never wait til the end of the semester, or, even worst, around application time to ask). Around this time, usually you'll have several quizzes and a test or two that you've done well on, actively participated in classes, and met with the professor maybe once or twice. So with all that stuff done, hopefully the professor notices you by now. When asking a professor for a LOR, I would usually ask in person. If they say yes, give them your PS, resume, or CV to make their life easier. If not, give them the middle finger. I kid! Just thank them anyways and awkwardly move on. Also just to put it out there, when asking for a LOR, it is best to ask for a POSITIVE / STRONG letter of recommendation. I have heard horror stories from other professors and students claiming their professors offering letters only to write negatively about the student (so not cool). It's also advised to go around and ask a few professors just in case one decides to back down later on. Two or maybe three weeks before the letter is due, send the professor a friendly reminder that you're about to apply to whatever, and you'll give them the necessary information on where to submit them. Once everything is said and done, it wouldn't hurt to send them a gift card or a thank you note to show your appreciation to them.
Depending on what the letter writer wrote, I would advise not to use the exact same letters you use for post-bac/masters to apply for dental school. If in doubt, I'm sure if doesn't hurt to ask your professors once again to revise your letters when applying to dental school. Better yet, you can aim to use letters from professors, etc that you will have during the post-bac/masters program.
Best of luck~
