Even if i used plywood i would still be required to have foam prior to the tank being placed on it, or i would void my tanks warranties...but again up to you
It levels out any uneven things (wether that is the wood intself or something that may get there on setup) that maybe under the tank which can cause glass stress leading to failures.
I had a stand for a 90g but I only put two 10g's on it. Definitely works for me cause it's two little tanks. I can even add another 10 but I don't need to yet. Idk about two 55's on a stand for a 180. I wouldn't do it myself but if you have enough support then do it.