prepare whatever information you think they might want just in case. a statement of intent and why dentistry (with some background to give yourself context) is definitely a good idea. you can probably ask them what information they might need from you. unless it's a new and young professor, he/she probably has written many recommendation letters and will have a good idea of information he/she likes to include in letters.
honestly, while your complete transcript might be nice, i don't think it's completely necessary to include it. they don't need to know that you got a B in Art History if they're a Bio professor. it's not really relevant. they might want to know your overall gpa, so it might be nice to include that somewhere. i applied through my committee and they had required that we answer a range of questions for their file, and that included calculating our own Tgpa, Sgpa, and Ogpa, as well answering questions about extracurricular activities, why dentistry, etc. prior to asking for recommendations. so i just essentially passed this info on, including any other additional information my professors asked of me (some wanted to see my old papers from their class). i think i also just threw in my resume for the heck of it (though the questionnaire was like a flushed out resume in itself).
i agree with some of the above posts. providing more information about yourself helps the professors get a more comprehensive understanding of you and your interest in dentistry, which can lead to a more robust letter. but it's ultimately up to you in how much info you want to provide. present yourself well, but honestly as well.
ultimately, i think the letters really boil down to asking the professors whose classes you performed well in and the professors who like you (i.e. will put in a good effort into your letter because they care at least even a little). if you did well in THEIR class, they'll be more likely to give you a good recommendation letter and list your strengths, even if you struggled in another class.
