Once you establish a drawlength it should not change. A loop does not change the bow. It only extends your anchor point farther back. This can be detrimental to your form. What you might want to do is get a release designed just for loops that move your hand closer to the string (shorter head on the release). True Ball's Short and Sweet is such a release. There are others. In a pinch you can compromise and shorten the bow a tad, and keep the loop as short as possible also. Many guys have too long a loop on the string. So long as your release dows not touch the arrow nock it's long enough.

I've always used a rope release so my "loop" is on the release. Same concept though keep it short so you do not overextend your drawlength.