I did my grandbabies quilts with a stitch-in-the-ditch and they were 2" apart and the quilts were not stiff so I would say that anything from 2" up would be fine and like Minda said just check the recommended spacing for the batting.
I'm just learning the FMQ also. I took a class a couple of weeks ago and it really helped with the tension, that is where I was really struggling. I hope to get serious about learning after the 1st of the year.