Always a loop.

Without a loop....
Take a nock off your arrow and nock it on the string all by itself.
Draw your bow and watch the nock spin.

Now add a loop and do the same thing.

Secondly....if you use a release and nock under the arrow, using a button or not, you have immedietly created an inherent downward angle of you string and the arrow at release.

Big reason a LOT of rest issues relating to arrow flight can be eliminated right away by adding a loop.