Ah, good point. I was thinking in that case of someone lifting the whole stem out and clamping it in a bench vice over a drop cloth, but a drop cloth over the bike would achieve the same ends. In my case, it'll be using a stem with a removeable faceplate as I shellac cork, but will have cables running under the tape, so yeah, I'll probably want to have it on the bike.

I do have to say I like the looks of the way that OP did it as it didn't cause the bars to mismatch the saddle in color, but I'd be afraid that the saddle will change in color and the bars won't.