Bruce Lange either asked or answered this question, attribution is hazy:

" I have a question about converting a 130mm Ultegra hub for use on
a bike

that's spaced to 135 mm. I know you can do it by adding a 5mm
spacer and redishing the wheel, but once that's done, how does the
wheel compare to a wheel originally built on a 135 mm hub in terms
of strength?

If you add the 5mm spacer to the left side of the hub, the wheel will
be stronger because the dish will be reduced and the spokes more
evenly tensioned between the left and right sides of the wheel.

Phil, Squid-in-Training wrote:

This begs the question:

Why not make all hubs such that the dish is zero, right off the bat?

Because if the flanges are too close together it makes the wheel weak
laterally.

Or why
aren't all mountain hubs have the flange spacing of road hubs, so as to make
it stronger?

They generally do have the same flange spacing, which is part of the
reason mountain-bike wheels are generally stronger than road-bike wheels.

Sheldon "It's All About Triangles" Brown
+---------------------------------------------------+
| Time shouldn't just pass; things should happen. |
| --Harry Turtledove |
+---------------------------------------------------+
Harris Cyclery, West Newton, Massachusetts
Phone 617-244-9772 FAX 617-244-1041
http://harriscyclery.com
Hard-to-find parts shipped Worldwide
http://captainbike.com http://sheldonbrown.com