personally I wouldnt use any kind of pressboard in an aquarium stand... because if pressboard gets wet it swells which could adversly impact things like your tank being level, and it weakens considerably...
yes I know you'd probably finish it with some sort of waterproof finish, but with the amount of water that I always seem to get on my stand, I just wouldnt take the chance that some could get into a spot I missed, or a spot I had to drill a hole later or some such thing.