Sorry for not replying sooner, I didn't realize there were more responses! I did finish the covers that night (a photo is on my blog at
http://bplo.spaces.live.com/blog/cns!CF2831C0AE64E81B!2289.entry but I haven't tried again (the room where I sew is under renovations).
One thing I realized is that I have been cutting on the floor--on a cutting mat, but still on top of carpeting, so that might have been partly responsible for skewing the cuts. The interfacing idea wouldn't work so well in this situation, since the linen is being patched onto something else and interfaced after that. But the freezer paper could work. I love the look of these so I will give it another go once the dust clears and I get the machine back out! I bought a lot of linen and have several other projects in mind as well...
Thanks for the tips,
Bonnie