I do recall seeing Leah Day make that recommendation. Her claim was that she thought it made a difference in thread tensions.
I've tried it and found it made no difference on my machine, and that the feed dogs up did damage the slider. I see no reason to leave them up since their purpose is to move the fabric and they have nothing to do with thread tension. They just get in the way when FMQ.
Even with the feed dogs down, I use a slider (actually, a $10 oven liner rather than the pricey Supreme Slider). I tape it down with painters' tape and it definitely helps reduce drag.
Last edited by mkc; 05-01-2020 at 02:20 PM.