Absolutely tie the lines long enough to allow for the fall in tide without inducing material strain on the lines or attachment points. This document has some info on the stretch characteristics of nylon, http://www.mazzellalifting.com/catal...ad/cordage.pdf
, which is longer than I thought! But if you secure your lines so the boat hangs on the lines, at all, you are engaging in an expereiment to find out where is the weakest link, and you are likely toi find out the hard way.
Secure the bow and stern lines to the nearest piling a little shorter than the distance between pilings (i.e. the boat can't reach a piling on either side. then add a bow spring line and a stern spring line each long enough to hold the boat in the center, they don't need much slack due to their length and angle. The boat should pretty well stay in place, be glad you arn't in Maine dealing with 12' tide drops.