Hi Cathe,
I did two quilts for a client using the McTavishing technique and both quilts draped nicely when I was done, despite the heavy quilting. I didn't micro-McTavish, which is really REALLY close together, but my stitch lines were usually as close as ¼" apart throughout the quilt. She chose a very thin cotton batting, and I used a very fine bobbin thread (60wt Bottom Line) and also a really fine (50wt I believe) embroidery thread on the top. So even though the there was a lot of thread in the quilt, keeping the batting thin and light as well as the thread made a huge difference. I was concerned at first too, that the quilt would lay like a board on the bed, but thankfully it didn't.
If you use the McTavishingi technique, you probably won't ever achieve a "soft, lofty" look with your quilt, but that doesn't' mean it will have to look stiff. You can do this technique as large or small as you want. Make it your own! No rules here. :) I saw a small quilt at a museum recently that was micro-McTavished and I was floored with how close she got that design-it ranged from 1/16" to 1/8" and in places the threads were nearly on top of each other. It was done on a miniature quilt, and yes, it was really stiff! LOL But for a tiny wallhanging I guess that would be ok. :) I think it's more important to use the design idea (the way Karen McTavish chose to make the lines flow) rather than worrying about how far apart each row is from the other. It's really a pretty flexible design. Just practice a few swatches and see what you're comfortable with. :)
HTH!