The question is - will you leave the dryer sheets on the string blocks when you make the quilt?
Con: Dryer sheets are highly flammable, so if your intention is to leave them on the blocks, well, not a good idea.
Con: Scent is another issue, as some people cannot tolerate artificial fragrance.
How about trying it with a paper towel that you can easily tear off when each block is complete? I have never done that, so, if you try it, let us know how it works.