I wouldn't bother with spacers. Any steel frame can handle a little flex like that. For years, everyone road 130 mm wheels in frams that wanted 126, and it worked fine. Shimano made their 8 speed Dura Ace hubs with rounded axle nuts to help spread the frame.
2.5 mm deflection on each side just makes getting the wheel skewer tight a little bit more of a hassle. Beyond that issue, there is no issue.