Not sure where you got your dowels, but I have used American Woodcrafters, and they came straight. BTW, I spined a bunch of the 5/16" birch, and they run in the mid-40's for spine.
As stated, flex each one before you put much work into them, because they are dowels, not arrow shafts, after all. If they were sold as birch, they probably are, since I don't think Ramin is imported into the country anymore.
Good luck!