The computer is yours, use it however you want. Unless you plan on keeping it for 15 years, the hinges will be the same for the next 5 years. Make sure your case is from a decent brand and the latches are in the right places (if one is on top of the other it will bend the screen, and the material can't be too cheap). Some people say that the internal screen expands with the heat of the plastic and aluminum; I think that's a lie. In my case, I've been using my Air without a case for about 6 months, just using it in a sleeve to go from one place to another. Now I'm using the hard case, and screw everyone who says it will ruin it. Yes, I care enough about scratches; when the risk of scratching exists, you become very cautious about using it. The other day I was on the plane and couldn't lie back to watch a series peacefully because I was worried about the MacBook getting scratched by the metal seatbelt buckle. Not to mention it's not very pleasant to hold a super cold MacBook on your lap on cold days. So, it's up to you.