While it's still possible the Steelers plan to look at a free agent to fill a common position on most teams (the 5WR), it isn't a necessity, necessarily.
Much of the offense Steelers offensive coordinator Todd Haley ran in Kansas City - his last coaching gig before arriving in Pittsburgh this offseason - saw a lot of passing to running backs and fullback/H-backs. While WR Dwayne Bowe owned much of the bulk of targets, there wasn't ever a receiver clearly earning second in that race. It was usually a running back.
This isn't an indictment on the role Cotchery - listed as the 4WR - will play this season, but rather, a 5WR spot isn't particularly necessary, especially when the Steelers made a point to draft a good pass catching running back in Chris Rainey (a lock for the final 53) and already have TE Heath Miller, who can split out wide as needed, and TE Weslye Saunders, who, upon the completion of his suspension, looks to continue improving as a receiver as well.
While Williams and Gilreath flashed some potential at times, the whole group's fate was pretty much sealed the minute Steelers linebackers began falling apart. At that point, an extra receiver was more of a luxury than a necessity.