New redwood deck help

Hi, I'm installing a new redwood deck in Southern California that is going to be in direct sunlight from about 8am to 4pm every day. My contractor has suggested sealing/staining the underside and sides of the redwood planks before they are installed because they will be inaccessible after installation. Is that necessary or recommnded (does the wood need to "breathe")? If I should seal the undersides/sides, I've read that you should wait a couple months for the new wood to weather, which isn't going to be possible for the underside/sides. Should I not stain the underside/sides at all? Go ahead and stain it brand new from the lumber yard?

Also, given my location and the conditions, would you recommend TWP 1500 or Armstrong Clark?

Thanks for your help!