<p>Sending a letter is always a nice idea. If there's a particular reason (e.g. finances) for your declining their offer, you may want to discuss it with them before declining. Sometimes schools surprise you and are able to come up with some funding to make their school more affordable. An e-mail would be another way you could notify them about your decision.</p>
<p>Honestly, Northeastern's so beaurocratic I wouldn't worry too much about it. You send the myneu thing, great, but otherwise if you don't deposit they know... Nice thought though.</p>