You don't really have to drop or cover your feed dogs at all - in fact, covering them may make it more difficult to move the fabric through smoothly, because of the thickness the cover introduces between the bottom of the foot and the needle plate.
I've forgotten to lower my feed dogs a couple of times and I really couldn't tell much difference. One time, I quilted with them up for so long, it chewed up my Sew-Slip mat - and even then, the only reason I stopped was that the thread was catching on the chewed up mat and causing skipped stitches.
Try FMQ without covering, first. :)
It's going to be more difficult with a 66 and 99, to begin with because of the bobbin configuration - no sense making it even harder on yourself. Especially if you're new to FMQ. :)