Another consideration: the dropouts. If the faces of the right and left dropouts aren't parallel and the slots inline with each other, they will apply a bend to the axle. This bend will cause real levels of stress that will add to the stresses that your (and your bike plus your cargo) weight(s) cause. This is the first place to look if the bike regularly breaks axles.

The good thing is that correcting the dropouts is easy on any steel bike (as long as those dropouts were reasonably correctly located in the first place). Take the bike to any bike shop. They will have a fancy pair of tools that clamp on to each dropout and are used to a) see how close the dropouts are to being parallel and in line and b) serve as levers to bend the dropouts to the best possible compromise. (Usually to perfect as far as hubs are concerned.)

Any bike should have its dropouts checked at least once, if only so you know. (On non steel bikes, correcting may involve other measures. Talk to someone who knows the materials.)

Ben