yes, they do disappear. This is precisely why you must take advantage and use them now. I have sent to schools I KNOW i will apply to. Try sending to two safeties, two reaches. One of the main reasons to send is to let the college know you are interested. They will start a file on you once they get the scores.
It's important to send to safeties. For example...perhaps RIT is your safety, but you are dying to go to MIT. Send to both in case you don't get into MIT, because at least you will know you showed RIT you are really interested.