If you're concerned about the smell of preservatives, head cement, etc. on your flies here's something that may help. About thirty years ago an Amish flyfisher I met on a Central PA. stream told me to always rub my subsurface flies on a slimey rock or in the mud at the bottom of the stream before I fish the fly. It masks non stream smells and helps make your fly more buggy and, he claimed, more appealing to trout. I sometimes do it, sometimes not. But, I asked about this practice on a local PA. flyfishing website a while ago and was amazed at the number of folks who said they NEVER fish subsurface flies without doing it. Most of them said it works very well.
However, I personally doubt it makes much difference in a fast riffle. My $0.02.