don't think ya need any in my opinion. fly fishing is presentation not smell. plus the fish don't smell a dry fly or most kinds of flies around they just munch um up. most river fish only have a few seconds to decide if they wanna eat or not. they don't usually have time to smell what the fly fisher is cooking.
. some people say even using a san juan worm isn't fly fishing or if you use a fly to imitate a regular earthworm. i say do what ever you feel. you got to look yourself in the mirrior the next day and say you fly fish.