I use whatever stitches correctly for the weight of material and batting - could be a 14, 16 or 18.
Just remember to start with a sample sandwich and PRACTICE! It really helps get stitch length down correctly as you move the fabric around the machine. Once it is working correctly, THEN go to the project (the same day) and do the FMQ.
Ask me how I know! I can't begin to count the number of hours spent ripping because stitch length wasn't right or my hands weren't moving right or I practiced yesterday but didn't FMQ till today, etc. It adds too much angst to the project when all you want to do is FINISH it.