I would have to say that, if you want to go into GP, then P/F schools will make d-school relatively more relaxing and enjoyable. However, I think for specializing, letter grades can be a huge benefit (ONLY if you do well in classes), because this will lead to more than 1 grade making your application (NBME grade now that the NBDEs are P/F).
For example, if two students are exactly the same in terms of ECs, research etc., and they both receive 92 on NBME, one person gets a P for d-school grades, while another gets 95% As in dental school, what looks better? Obviously if you're getting 95% B/Cs that will not help.
To build on that, focusing on grades can be stressful, trust me. I was thinking, though, and I realized that If you want to go into GP like this poster stated, then P/F schools are nice, but at a school with grades, passing with a 70% or whatever passing is, is the same as receiving a P. So if this is your outlook on dental school, either school should be fine as to get a P, you'll need to perform the same as you would need to to get a passing grade in a non-P/F school anyway. Just with a P/F school, you don't have to worry about it being seen that you didn't perform on an A-level by as many people.
I've spoken to professors at pass/fail schools and they still keep track of students that do well in their classes, even though it's pass/fail. With this, they will write you letters of recommendation if you were one of those students who excelled in that pass-fail environment.
Either way, a school with grades will foster more competition amongst your classmates, but if you have a class like mine, no one lets it interfere with interpersonal relationships between each other and genuinally want to just do their best to keep options open and be the best they can be, and not so much, better than you (though this inevitably goes hand-in-hand).
That being said, as I am in a non-P/F school, I'd be fine either way. If I did want to use my academic record in school for options in the future, I'd prefer to not have to get letters from each professor to show I have put in my greatest effort. Also, it's hard to measure your greatest effort, when you don't have grades as indicators.
Hope that made sense.