Chain tension adjustors like the ones shown on this page can be helpful to keep your rear wheel centered and at the proper chain tension while the axle nuts are tightened. Although they're not a substitute for properly tightened axle nuts, in some instances they may also help prevent "pulling" a wheel under high torque loads.

MKS' newest generation of tensioners (photo above) have adjuster bolts with 10 mm hex heads which are externally accessible. There are versions with 5 mm end caps designed for classic steel forkends like the one in the photo above, or with 8 mm end caps for aluminum frames.