The best way to seal a deck is to dip the boards before installing them. This will give you 100% protection. This is very rarely done by the contractors. The only decks we clean and seal on the bottom side are what we call walk unders. We do it at the owners request and we charge accordingly. The sealer that you use must be breathable. There are several reasons why cedar will rot out. The two main reasons are #1. The deck is built to close to the ground without a moisture barrior and #2 is that someone painted it. The other reasons are mostly due to construction . Cedar and redwood must be able to breath. When these two types of wood rot from the center out, it is a sure sign that something has been put on them that will not allow the wood to breath. Moisture raises, moisture comes up from the ground, collects on the bottom side and penatrates into the wood. The top side being warm, wants to draw the moisture in deeper and expell it out the top. If your sealer won't let the wood breath, then the moisture will sit in the middle of the board and eventually rot it out. The same thing holds true with other species of wood but cedar and redwood are the most prone because they are a softer wood. Hope this gives you a little insight.
Jim Bilyeu
Exterior Woodcare