Rear wheel at edge of dropouts on a Surly Cross Check The rear wheel on my Surly Cross Check is right at the edge of the rear dropout but the chain is tensioned correctly. If I move the wheel back into the dropout so it is comfortably in the middle, I presume that the chain will no longer be tensioned correctly or will it? Does the rear derailleur compensate for wheel shift fore and aft?

If not will I need to get a longer chain? I asked the bike shop but they didn't seem keen on moving the wheel back into the dropout. They just said the rear dropout screws were correctly configured.
When you have a multi-speed setup, the derailleur handles all responsibility of maintaining chain tension. 

The Cross-check frame has the semi-horizontal dropouts so it can be utilized for single speed applications, and it's only for those applications that you control chain tension by wheel spacing.

Where the wheel goes in the dropouts for a multi-speed setup is not super important, so as long as it is completely "in" them, and there is plenty of surface on the dropouts your axel nuts/QR cams ares secured to, you should be fine. Having the wheel nearer the end does make it easier to pull the wheel out without getting too much grease on your hands.