I was a BPI Certified Building Analyst in Austin, TX back in '09, and couldn't stand taping registers anymore. Not only is it slow, annoying and cumbersome, it's expensive, wasteful, and often damaged people's property. There I was in the Austin heat with the a/c off, sweating like a pig, and dragging a ladder through a furnished home trying not to run into anything. Rushing around, with my hands above my head, slathering disposable plastic tape all over.
More times than not I was trying to seal condensation covered, dirty, or even freshly painted supply vents. If the duct mask did stick, I'd risk pulling off paint and wallpaper (and being charged). Seemed like a whole lot of work to perform - on a ladder - all for a 30 second fan test. Especially since you then have to go back and rip it all down?!
I couldn't stand it anymore so I came up with my own reusable system which I've been perfecting over the last 5+ years, adding products along the way.