Simple rules:
Forend, the grain needs to run straight or turn up slightly near the end. Not only does it look better but it also contributes to a more stable stock.
Grip (most important area), the grain can run straight through this area (looking from the side) but idealy it needs to turn down a little. More importantly, you need to make sure the grain doesn't turn out anywhere in this area looking at it from the top. This is where most stocks break from bad layout. Just make sure you have a nice straight grain line from the back of the action to the start of the butt.
Butt, this is open to what you like. The only thing I try to stay away from is blanks that are laidout with the grain turning up in the butt area. These never look right to me when finished.
Nice layout: straight through the forend, turned down in the grip and the butt turns down a little more.
Bad layout: grain turns down in the forend, cross grain in the grip area. Bark pocket in the butt area.
While a badly laid out blank can be used successfully (I used the one in the example above) you'll get a lot nicer stock if you bypass it and sacrifice a little figure for a better layout. You'll hear the comment many times "grain doesn't follow figure" this is true but,,, except for the base of the tree (butt area) figure does pretty much follow grain.
Just remember when shopping for a stock blank, grain flow on the top and bottom of the stock are as important as what you see on the sides and straight is best. it'll also give you an idea of what's on the iside of the blank.
IMPORTANT TIP> The more figure lines you see on the top and bottom of the blank the more figure you'll see after the stock is turned.
Last, there are three different cuts when it comes to stock blanks; slab, rift and quater cut. All three can make a great stock but there are differences. Most of what you'll see will be slab cut.
Slab cut: They get more blanks out of a tree with this cut but it will also create layout problems a lot of the time. Sometimes trying to figure out what a slab cut stock will look like after it's turned can be nothing more than a complete guess. A plain stock blank may have a lot of figure on the inside or it may have next to none. You can also get "one sided stocks" where one side will have nice figure and the other will look plain or have much less figure. IMO, slab cut stocks are the least stable because they tend to move from side to side and not up and down. Your chances of getting a finished stock that both sides don't match are greatest with a slab cut blank.
Rift cut: These are a cross between slab and quarter cut and are sold as true quarter cut most of the time. The nice thing about these is they very much resemble true quarter cut wood, are stable and what you see on the outside is usually on the inside. Usually your better pieces of 1 piece rifle wood are rift cut.
Quarter cut This is a rarer cut than most people realize and mainly used on two piece stocks where the seller can get enough stocks out of the tree to justify the cost.
This is only a start, but it should help you get started shopping.
Terry