She does leave them up, so I followed her recomendations with the foot and the dogs up, and I have never had a problem.
It does dent your 'Slider" a little if you are using one. If you have a single hole plate, that helps, too.

Leah Day keeps her feed dogs up ... like you, she discovered it by accident.

The purpose of FMQ is for the sewer to be "free" to do the motion instead of motion being controled by the machine. By lowering the feed dogs and using the FMQ foot the sewer is in control of the quilting. Many times the eyelashes/loops are caused by the sewer's speed of moving the fabric. By leaving the feed dogs up they put a little drag on the fabric therefore slowing down the movement of the fabric. this slower speed may be preventing the eyelashes/loops. The drag on the fabric actually prevents the fabric from moving "freely" and may make you FMQ harder. Also if you are using a slider then why leave the feed dogs up since covering them will stop them from grabbing the fabric. as for damaging the machine? not sure but it can not be the best thing for it since you are working against the feed dogs. There may be a good reason that lowering the feed dogs is recommended. IMHO

I leave the feed dogs up, use 0 stitch length, and use a super slider on the throat plate. It works well for FMQ for me.

I had the exact same "accident"! Now I always leave my feed dogs up.

Hey whatever works for you! If it was messing up your machine you would definitely know. Just make sure you clean your machine as often as need. I clean the lint at the end of the day. Also if I'm going from one different type of fabric to the next.

I think that Leah Day says to keep your feed dogs up for FMQ.

Leah Day always quilts with the feed dogs up.

I have read that on certain machines the feed dogs are somehow connected to the tension. If that is true (and it sounds like it is), it would affect the stitches on FMQ. It really is a matter of what works for you. The important thing is to keep trying.

I've done it with them down and up. I took a class and the instructor said to leave them up. worked. but then, there are times I have difficulty moving my quilt and can't decide if it is my fabric, batting or feed dogs. they are usually down.