That particular tank has been up for around 5 months, I think, with water in it. The mat has kept things level quite nicely. That tank is also rimmed. I'd tried to find a foam sheet to put under the glass inside the rim on the bottom, but they were way too thick and I didn't think it would compress enough for the rim to touch the top of the stand. So I went with the leveling mat with a double-layer. So far, so good, and nothing seems amiss.
Here's a photo of the tank setup where you can see the mat under it. No water in it during this stage of the build, but it has been fine since I've filled it.