Don't know about Dayton, but here, just about every non-Cleveland market station encodes, and Cleveland is the only PPM market in Northeast Ohio.
Since PPM became currency in Cleveland, you'll see the same Akron/Canton stations you saw before.
Since PPM became currency in Columbus, you can even see stations like Wooster's WQKT/104.5 in the Columbus ratings, so I presume they're encoding as well.
From what I've heard, Arbitron basically hands over encoders to EVERYONE, even in adjacent markets and even small non-comm stations.