Personally, I'd coat all of them with a sealer. Both the soft wood and the MDF will absorb water, and there are not too many spots that have more water available in the air than on top of an aquarium. I've used softwoods (specifically pine) to build canopies, and it can be done, provided you get several good coats of a sealer on them to keep them from absorbing water. The MDF, I can't say, but I personally would be pretty hesitant to use it.
"Listen to some of these guys talk, and it's like they were born from their momma's belly with a fishkeeping encyclopedia in one hand and an API kit in the other" (unrevealed).