Installing tile over hardiboard
I spoke with someone at Home Depot, and they made a good point - You're screwing it down on top of the thinset, and therefore you're already on top of the board. The more important part is to let the thinset on the seams (along with the tape) set up for a while (a few hours) before tiling. We did just that, and so far so good - the tiles went down fairly smoothly, though I still need to grout.