As noted the qr skewer faces carry most of the load but a slightly short axle will not overstress the dropout. In fact, it is common to respace a 126 mm hub to 130 mm which reduces the axle protrusion from 5.5 mm per side to 3.5 mm and it causes no problems.

Here is Sheldon Brown's comment on the topic:

"Overall axle length for quick release hubs is commonly 11 mm longer than the overlocknut distance listed, 5.5 mm on each side.
In practice, the axle can be quite a bit shorter than this...even 1-2 mm protrusion past the locknuts will suffice to locate the axle properly, so, when converting a hub to the next wider spacing, it is usually not necessary to replace the axle."