When I am looking for seat material, there are a few factors that come into play. First of all, it should be uneventful wood, from a straight tree that has been sawn straight along the fibers, or as close as possible. I know that in flat work, highly figured wood is prized, but I'll be carving into this wood, and the more settled the grain, the more the grain will highlight the contours.
My other concern is width. Most of my seats are just below 20" across, so I look for two basic widths to conserve material. Full 20" wide seats are great, but they are tough to come by, expensive and because they tend to come from the area nearest the pith, they tend to warp more than I like, leaving me some delicate planing to get the final thickness.
My second choice is a 10" plank, and I'll show here how I crosscut, split and assemble it to match the grain. Here is the plank, crosscut for the length that I need into two pieces. Then I split one piece and roll the halves outward as you can see the arrows directing.
Here is the position after I roll them out.
Then I slide the other piece up between the halved piece and that is the orientation of my glue up.
If the board is relatively straightgrained, you'll see that the angle that the growth rings leave the center piece will be complimented by the way that they move into the outer pieces. Below is the piece glued up.
This method moves the boards around in such a way that I often find that I can't plane them all the same direction on the face. But I find this to be the case with most one piece seats anyway. This is caused by sawing a straight board out of a slightly twisted tree. But whatever the issue, it almost always subsides when the carving starts, because the angle of the side of the seat bowl cuts across the fibers anyway, regardless of whether they are subtly ascending or descending.
Here is the planed seat.
My goal in this is not to completely erase the joint, but at least to make the viewer have to look twice to see it. It's just one more spot where a little care and craftsmanship can enhance the whole impact.