I'm actually dealing with/debating the same issue right now. MDF vs. conventional wood. Started putting together parts for a 50, and wanted to do it on the cheap. So I picked up a used tank... and ordered the cheapest stand I could find for it. Well, the tank had a chip, so it's not being used (ordered a brand new reef ready 65 instead), and I STRONGLY considered trying to return the MDF stand and just buy a wood one, as the budget for this tank is totally out the window now anyway. But after getting the MDF stand put together... it's actually really nice. Boards are sealed well (although I may use the caulk around seams idea), the design is a little nicer than the other wood stand I have for my 20 gallon, and it feels much much more solid than I was worried it would. I hope I'm not making a big mistake here... but I think I'm going to use it.
I apologize this was all a little off topic... but back to the OPs question.... at least in my opinion, this hobby is ultimately about relaxation and enjoyment right? So while I think that yes... you're almost certainly fine with the 30 inch tank on a 32 inch stand (I'm no engineer.. so don't put much weight in my opinion please lol), if it's going to make you more comfortable to put it on a sheet of marine plywood, or 2x4s, or scrap the stand all together and buy another one... well then that's what you should do, IMHO. Regardless of what anyone else says... you need to be able to look at your tank at the end of the day and not be worried about it. So if you're going to worry... do whatever you need to to alleviate that. Just my $0.02