Cedar Deck boards
Your idea of applying a sealer to all sides of the material prior to installation is not a bad idea. How far is your deck going to be off the ground? Will you be able to maintain a finish on the underside or not? If not, sealing before installation on all sides, and then sealing cuts and end grain will certainly improve the woods ability to stand up to the environment. The underside of the wood at any given times is around 2% more moist than the tops.
Even if you won't be able to maintain a finish on the underside, sealing it all won't hurt. However, make SURE to use a penetrating oil finish after properly cleaning the wood, and allowing it to dry thoroughly. Typically on new wood we will clean it, use a brightener to bring the woods natural color out, and give the wood the proper ph for staining, allow dry time, then stain with a penetrating oil stain.
To stain all sides, lay out some saw horses or blocks to hold the boards off the ground, spray and backbrush the stain on, wait 1 hour, flip and repeat.
I could write a book, but I hope that helps.