Anyone want to comment on applying just to my Thermarest?
If you are planning to sleep in a tent, such that you might want to position yourself a few inches to the side or front or rear depending on the bumps in the ground, then putting some sort of sticky stuff on the thermarest might be better, since otherwise you would have to extend the strips across the entire surface of the tent floor, thus adding weight. For a thermarest, seam-grip would probably be better than silicone.
If you are using a bivy, then normally the mattress always goes in the same exact position and you shuffle the bivy around according to the bumps on the ground. In t his case, it is better to put the sticky stuff on the bivy. In particular, you can trace a strip of sealer where you want the top of the thermrest or other mattress to be, which makes it simpler to get your mattress in the proper position at night.