If a string gets twisted too much it becomes something of a spring and you can lose performance. I don't remember how many twists per inch is recommended so I hope someone else replies with that.
When you unstring the bow just leave the bottom loop on the nock and run the top loop down the limb. That way the string can't untwist. Also, if you wax the string and rub it in well it will tend to stay as it is if you take it all the way off the bow.
You don't mention if you have a glass lam bow or an all wood bow. If it's all wood you should unstring it when not using it. For a glass longbow, there are some people who leave them strung when shooting frequently and others who unstring them when not being used. Personally, my transportation to the shooting area and storage situation when at home has me unstringing all my bows after shooting is over.
Guy