Roy - Care to elucidate on why 1/4 is the best for bow making?
Casper - When you go to select a board my advice is to look for the straightest grained piece you can find by assessing the straightness from ALL 4 sides of the board. Try to get a feel for exactly how the fibers run through the board from end to end.
If you get a straight grained board go for it.
An important factor with ash is the ratio of earlywood to latewood. You want nice fat rings with the smallest amount of porous earlywood.
Also crown your sinew so the back ends up convex. Plenty down the middle with little on the edges.