if you are going to use a flat or plain sawn board, (which is the orientation of the growth rings you have), then it is best to have the ring lines on the edge of the limb straight as possible. Those "flame tips" stepping down the back are a dead give away.
grain may not always run straight parrellel with the length of the board on account of sometimes grain spirals up the trunk as the tree grows, or the board may not be cut from the log true. this condition is hard to see from just looking at growth rings. a board that splits on a diagonal is an example.
one thing for sure though is that grain will not dive up and down through different years growth rings
I have had good luck with buying a split plain sawn board that had straight ringlines on the edges and ripping 3/4 by 3/4 strips parallel to the split, then flipping the pieces and gluing them together so that the edge rings are on the back and belly. if the board is sawn this way it is called quarter sawn or rift sawn. but not commonly found in the big box stores.
of course a wide board sawn from the middle of the tree will have quarter "sawn'\" ring orientation at the edges and a flat/plain sawn center.
