You could not decline the ED offer and then accept an offer with a similar financial aid package, for example. If it really is the case that the financial aid package turns out to be short of what you can really afford than you can decline but because you should know ahead of time about what to expect, that should not happen. The working premise is that it won’t happen. So while there are bound to be some instances of it, I would not apply ED to play my odds without being sure that if you got an offer you’d go.