.........which is why you shim as much of the perimeter as possible. I would never shim it at only one or two points, I would try to shim any part of the stand that does not contact the floor. My floors are level, and I have w/w carpet and pads, so low spots and such have never been an issue for me. Where possible, I believe it is preferable to shim the tank rather than the entire stand.
My point is, if you are going to have four legs that are the only transfer points to the floor, then you probably need bigger stock than if you are skirting the whole perimeter of the tank. I've looked at a lot of cabinet-style commercial stands for 75g, 90g, 125g, 180g tanks, and have never seen them use any 2" lumber.
If the top of the stand is flat I don't want to put shims there. I prefer to shim the cabinet. All I am saying is you don't have to shim the entire perimeter of the stand. You get the corners and at least one point in the middle your silver. Two points in between the corners and your gold.
That cheap building grade white or yellow pine will twist when it's not fully cured. That being said there are methods to building cabinets that will keep them from warping in any way. You do not need to frame out the stand with 2x4s you only need to use them for vertical and some horizontal support. See the stands I build on my signature none of them where framed out and they are fully squared.
Done right you never need 2x material in a stand.