It's a little of both, to be honest. If I have a commission I may search through blocks to find the right block for a shape. But if I am just making inventory I may see what I think the block wants to be based on the block shape. Sometimes I am looking at the grain, but sometimes I am looking at where the rings are because I can see it is going to be a great blast. So it varies, a lot.

Most new carvers are encouraged to first start carving just billiards until they get that shape down. It's got a certain amount of nuance although it appears rather simple. It's a foundation for a ton of other shapes to jump off to. That being said I first sand the sides of the block to get a good look at the grain and the growth rings. I look at the grain and try to figure out what shape will best highlight it within the constraints of the block shape. If I'm not seeing something I like, I'll then do the same looking at the growth rings for a sandblast. If I'm still not seeing something I want to make I'll move to another block and revisit the first block later with fresh eyes. I buy the biggest blocks available because it will more likely be from an older plant and, if possible, I try to get more than one pipe out of it to cut down on cost, but it does get tougher to figure out what shapes to carve. The more shapes you get under your belt, the more options you'll have to make the most out of the block. Same with moving to a shape first rather than a drill first technique.