Can I use these letters of reference later?
I suggest that if you want letters that won't become relevant for a few years, that you tell the writers that so that they might be able to update their recommendations at the appropriate time, without forgetting you. Keeping in touch is also useful for that.
I don't know the procedures at Waterloo, but some universities require direct transmission from the writer, not the candidate.
Whether they help or not depends on too many things, including the unknown future, to comment directly. But having the &quot;on the shelf&quot; is probably useful.