Secret to long-lasting relationship: Shut Down or Close Lid? The guy at the Apple store was like: "I just close the lid, and that works fine for me. I also shut it down every 1 (or 2) weeks, to clear the RAM".

That makes sense, but it sounds like his personal experience. Is there anything official/unwritten law on what we should do when we are done with our Macs, at night for example.

I am a person that almost accesses the laptop (MacBook pro 15'' Retina) every day. It's brand new and I wish for a long-lasting relationship with it!
On the software level his advice should be fine. The only difference could be battery life. Hibernation still consumes a tiny bit of energy and is therefore using the battery. Shutting it down completely should make your battery last a little longer which means that you're using slightly less recharge cycles. But let's be honest, the difference is near to zero - so don't worry about it. His advice should be fine.

A bigger concern should be your screen. If you clean it too often the reflective coating is going to get ruined. And if you don't use a keyboard cover you can ruined your screen/coating too.