Hi Dave,
do you mean planing the boards before ripping or planing off saw marks on the individual strips? I would definitely plane the boards, will make even stripping of the hull and deck a lot easier. About the latter I don't care, I use a finish cut blade from festool and it doesn't leave any marks at all. After all, hull and deck will undergo further planing and sanding anyway...

Concerning humidity I think 19-20% should be ok for ripping (although this will wear out your blades a little faster). Never used Redwood but WRC has never caused any trouble in humid environments. just make sure your hulls are down to 10-13% before putting any Epoxy on.
I wouldn't worry about strips snapping, can't even remember when this happened to me last time.

cheers
Matt