It all depends how much of a lift you have with the FMQ foot (or I'm assuming here, your darning foot). Hand walk your machine through a stitch cycle. If your ruler slides under the foot at any time, it is to thin. But the worse damage you will do is break a needle.
The quilting foot mechanisms on the quilting machines barely rise during the stitch cycle and the foot, for the most part encases the needle completely.
If your machine is good then try it. I do it all the time with my Juki and get perfect straight horizontal, vertical and diagonal lines.