when do I sign the MPN

You should go ahead and sign the MPN and send it in, assuming that is the lender that you ultimately want to borrow from. Whatever lender you decide on, go ahead and fill out your paperwork and turn it in to your FAO. When you are accepted, you will already have the road paved for your loans.
You will be in control of how much you borrow, because before the school can certify and guarantee your loan, they have to send you an award letter telling you how much they will allow you to borrow, which you will sign as it is, or modify and send back. So the control remains in your hands.