Do fuel tank locations on the wing cause a potential for weight imbalance?
YES

If the port and starboard tanks drain at different rates in a gravity fed system (due to a blocked valve or a clog in a fuel line), then eventually a differential in remaining fuel will drive a lateral imbalance in the plane's weight.  Another (and better) way of saying this is that the center of gravity will no longer be aligned with the aircraft centerline.

I can't comment on how this situation might emerge in the P-26 specifically, as I could not find a detailed enough description of its fuel system.

It is worth noting that the same is true with fore/aft center of gravity drift in aircraft where the tanks are located entirely in the fuselage (or under it).  This often has a more dangerous effect if not properly managed (and as a result the Cg drifts too far aft).

It is also worth noting that lateral asymmetry can also be created by non-simultaneous release of stores (like the 200 lb of bombs that the P-26 carried).  However, in the case of the Peashooter, the bombs were carried very close to the centerline, so the asymmetry would be minimal (as compared to that on a P-51 for instance).