It is the stems that have 'memory'. If for instance you simply crimp a hackle stem to shape a realistic reversed hackle barb mayfly leg, ala Blades/Jorgensen, or a woodduck or similar feather in the same way, it will eventually straighten back out when it absorbs humidity. If you heat crimp the stem with a flame heated needle tip, it will stay put longer. That is why when I tie my realistic legs I , after heat crimping them, I touch each "knee" with a tiny bit of cement because even heat crimped stems will straighten somewhat in a humid area . I mention this if you are using the feather'with' stem still attached to the barbs.
Yes, you can get the curve out as Ron says and drying but I would think you would use the natural curve to your advantage. Also, but not always, you are using,as has been said, a section of the barbs rather than the stem and if you match them up curve against curve you should be fine. Or you should find the closer fibers to the tip straighter...woodduck mayfly wings for instance..most of the curl is in the lower half of the feather..which you wouldn't use for mayfly wings anyway, but as wing buds on nymphs..