Definitely. If you don't they will get the idea that you accumulated a ton of letters and just picked the best ones. Also, your evaluators may not feel like they can be completely honest when they know that you will be reading it later.
Yes. I accidentally did not waive it at first, and was told by one of my professors that dschools will look upon it poorly because they won't seem as credible. Also, it was my professor's policy to only write letters if we waived the right to view it.
Professors who notice that you will view what they wrote will perhaps 'fluff' the LOR to make you fell happy inside. But on the dental school side, this 'fluff' will look like BS and feel less genuine.
Just think about it...if the acceptance committee sees that you didn't wave your right, they'd be less inclined to feel like your letters are genuine. Your professors can't be completely honest unless they know you won't be looking at them.
If I was on the committee I wouldn't even read them if you didn't waive your right.