you will want the grain to be perpendicular to the bow.
i usually spine all 4 quadrants.
theory #1- when you find the stiffest spine- and assuming your weight is hanging from the arrow-then the opposite side( or the inside of the curve created by the weight- should be against your bow, because this is the way the shaft will be making its first bend as it moves around the bow(archers paradox)
theory#2-it doesnt really matter which is the stiffest of the two sides, as long as the spine aginst your bow, is the spine measurment that works for your bow.
spine both sides and use the one that is closest.
more importantly is to orientated the shaft, so that if it does have any grain runout, and if it breaks on a growth ring, the back of the arrow is going to lift up and away from your hand, not down and towards your hand- and in turn impale your hand.
to do this the easy way- look for the feathers or points created by the growth rings running out- and the points should be pointing forward on the top of the arrow when it is nocked- in other words, the side against your hand should have the feathers pointing back at you.
only problem will be if you are shooting arrows with a full length taper- you might not have the option of choosing the stiffest or best side for against the bow- as the arrow can obviously only go one way when you orientate the grain run out "points/feathers" properly.
if your shafts are perfectly true - with no runout- well then you wont have any problems!!
spruce is a notoriously straight grained wood- with the highest interfiber strengths out of all the arrow woods.
thats why all the top wooden sail boats with wooden masts will use spruce- lighter and stronger than fir, and less apt to tear apart.
and none of them use P.O.C.
and those guys put a piece of wood through its paces!!!!
obviously the other woods work- and have for a long time- just that spruce has better properties.
i personally like to use a lighter shaft with a heavier point- better foc!
but enjoy making the poplar shafts- it is a good wood, and you will have a blast making your own and shooting them