What kind of species are you using? I would use something stable like white pine. Twisting and warping will be a potential problem, but good storage practices will help with that.
I would plane timbers before you do joinery. Otherwise it could affect fitting.
I would space stickers about 2' apart. And make tall piles so there is a fair bit of weight on the beams.
I guess they are not going to stored in a building?
I would keep them a couple feet off the ground, preferable protected from rain splashing on the sides.
I hope that helps...