Installing expansion tank at washing machine valve?
So long as you do not have any check valves between the hot and cold supply, an expansion tank can be located anywhere in the system. 

Thus, if you have a check valve or PRV (which acts effectively the same as a check valve from the system point of view) on the main inlet to the house, you can put the expansion tank wherever it is convenient if there are no other check valves in the system. All the water (hot and cold) experiences the same pressure throughout the system, and the expansion tank will work wherever you put it on the system. 

This appears to be a widely misunderstood concept, as can be seen in the comments.

Note that if you put it on the far side of a valve, (i.e. not a check valve) it will not work when the valve is closed (but that will also make servicing it easier, if needed.)