Is it absolutely necessary to plane the strips if you're careful when ripping and you are going to bevel instead of B&C?

What about moisture content? I was going to use some redwood in the build mixed with the WRC. I contacted the mill and they said the wood is air dried, but only to about 19-20%. I don't want the deck to twist all up as it shrinks. What I have read is that Redwood is pretty stable like WRC. I thought with that moisture content it would keep the pieces from snapping. It's a mixture of Sap and Heart. Thanks