How can I maintain this rough line?

There are differences between the actual path, the brush preview and the actual final rendered stroke. If you want a rough stroke, use e.g. a natural media or art brush. Otherwise you will have to draw your spiral from separate segments, not a contiguous path to keep the jagged appearance. Of course there's a million inbetween methods as well like subdividing the path and jittering up the points ever so slightly. realyl depends on what specific look you are after.

Mylenium