Loose is better !
I think the answer will depend on how much space you have but my own view would be to allow them complete freedom of movement.
A cow that can move around and interact naturally with others will be happier and in many cases provide more milk, have fewer health issues with a knock-on effect with better fertility.
I have had experience of both tied and un-tied and would always vote for un-tied every time !
There are benefits for the stockman apart from increased yields etc, it is a lot easier to spot any bulling cows for one thing or spot any illness.
Good luck !