I quit selling due to back problems. I couldn't lift them and do the service work needed for customers. I still have mine and quilt often on it. To answer your question, the sit down model is much like your own sewing machine, it is mounted in a table and you move the quilt. The machine does not move.

What do you want to know about using a double needle?

I went to the HQ University in Utah as well (I have a HQ16, but have it set up on the large frame) & they definitely said that the machine was intended to stitch left to right. They said that moving right to left is possible, but increases the likelihood that your thread will shred and your stitches won't look as nice.

I quilt using a Juki and go in all directions. The underside stitching looks ok to me. I wonder if mid-arms follow the same rule as long-arms?

I use a Tin Lizzy too! I go however I feel like! ESPECIALLY in circles!

I think "bull chit", too!

You can "think" whatever you want, but if you watch the needle on a regular machine or long arm, the thread makes a different size loop when making the stitch, this is what causes the thread to shred or break or not make a "good" stitch.
As I said before, you can go in all directions, but the emphasis was to go left to right, I think this shows up more on the pantograph designs than on free motion quilting. This was told to us by not only the person teaching the long arm techniques, but the engineers as well. I took a half day session with the engineer that designed the machine.
I often stipple in an all over direction in small areas at a time, I don't see much difference in the stitch, but if I am doing a stipple going from right to left, I can see some difference in the stitch formation. Possibly I see it because I was trained to see it as I do work on the machines, and still do all the maintenance on mine. Since I look for other things, I am probably looking more closely, and am more critical of the stitch formation than others would be.
However you stitch, the long arms and mid arms are a great boon to quilters every where. I would never tackle a queen or king size on my regular machine, too much strain on my neck, arm and back muscles.

This is true for some machines. It has to do with the orientation of the bobbin. It has been a few years since I was a LAQer so I can't remember the details. It was possible to do circle and curliques in all directions, but the stitching did not look as nice if the whole row was stitched in the 'wrong' direction. That may not have been the reason for the person in the quilt shop that started this discussion, but it is still true.
If you are free motion quilting on a home machine the orientation of the bobbin may determine if you can free motion quilt at all.
Debra B.

I looked tonight and it is better from left to right! Even my circles are better as I circle from left to right! I guess it is my saving grace that when I take it off the frame it seems to all even out! Still love it!

So, for a mid-arm, quilt from left to right behind the quilt, and right to left standing in front of the quilt?

No, the length of the arm is irrelevant. Ask your vendor or do a test and look at the stitches. Perhaps one machine will do fairly well in both directions. It is up to you what quality you are happy with. The issue is a mechanical one, likely the orientation of the bobbin.