In the past few months, I've heard about a number of grad school applicants who asked their recommenders for copies of their recommendation letters, and I thought I would share my advice on that subject: Do not ask to see your letters. That request is not kosher, and it will rub a lot of recommenders the wrong way. In fact, if they haven't yet submitted their letters, that request may taint what they write about you, or they may even be offended enough to rescind their offers to write you a letter at all.
What is OK is for your recommender -- on his own initiative -- to volunteer to share a copy with you, either as a courtesy ("Here's what I'm sending, thought you'd like to know") or with an express request for your feedback ("Here's a draft -- am happy to make any changes, let me know.") Do not under any circumstances ask for or suggest modifications except when expressly invited to do so.