As far as I know, it's kind of indiscriminate who he lines up against as he usually does, as you said, have a 'side of the field'. His presence diminishes the value of any/all WRs facing the Raiders, but the fact of the matter is that the Raiders are pretty brutal against opposing WRs anyway (4th-best in the league against opposing WRs in terms of fantasy pts. allowed), so I'd be more worried about the matchup in general.
All this said, Wallace has been on quite a hot streak and will probably find a way to at least get you enough points to warrant a start - as the Raiders have not faced many guys with his speed who can truly take the top off the defense - so you probably have to throw him out there and hope that the Steelers find a way to get him the ball early and often and that he gets most of the 15.2/game they allow to opposing pass-catchers
