Why vertical grain? I sat here for awhile asking myself that very question. As usual, Google provided an array of interesting answers, most of which were in line with why I like vertical grain wood for paddle making. The big reason being that dimensional lumber, like a 1×10 piece of western red cedar (very expensive by the way), in a vertical grain cut is very stable. Pieces cut from that vertical grain board generally do not cup or bow very much (if any) at all. So I trust cutting blade pieces from a vertical grain board and both using them and selling them. I use them because I trust vertical grain blade pieces. I sell them because I trust that they will remain flat and not provide a bad experience for my customers. Generally no good ending to a story involving a mad customer with a piece of warped wood in their hands. I’m OK with quarter (flat) sawn wood for the shaft strips. The form takes care of warping and the one inch width of each strip does not leave much to work with for the forces that cup and bow.
Some wood, like walnut, I cannot find in a vertical grain. Walnut offers a unique color in the wood working world. It’s been good to me so far, but the risk is that the quarter sawn blade pieces may cup. Getting the wood under fiberglass and epoxy asap helps minimize this, but that’s not always possible.
I still use quarter sawn wood on occasion and it works. But I worry about it.
Vertical grain wood remains my preferred cut to work with for paddle blades.