crack on top of laptop, how concerned should I be?

[LunaSea]

5. The problem with adhesives

is the speed at which they break down. The sticky part of most tapes will turn to gummy goo in just a few weeks depending on environment, humidity, etc. You can change it out periodically but beware of further stress on the surrounding material.
Several companies make printed vinyl covers for the back of screens which should be much stronger than most tapes.
A cheaper solution might be a vinyl drawer liner or heavy duty sticker, but test it periodically for breakdown by peeling on a corner. I suspect you could find a blank sticker from a printer who makes bumper stickers. I have a company logo sticker from 2004 on the bottom of a laptop that is scratched and faded but shows no signs of gumminess.

If you are handy with such things, find a piece of stiff aluminum to cover the hole/crack, and cut a piece of vinyl or sticker that leaves a border of about an inch or so all the way around to further protect from damage. It would provide better structural support for the fix and should last a longer time without stressing the crack.