I was reading the installation instructions for my ThermaTru entry
door found at:

http://www.thermatru.com/pdfs/instal...nstruction.pdf

Besides the standard level, plumb and square checks, they also
mentioned this:

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Check to be sure the framing walls around the opening are in the same
plane. Do this by performing a “string test” for plumb.

String Test for Plumb: Attach a string diagonally across the opening
from the outside, as shown. The string(s) should gently touch in the
center, if not the opening is “out of plumb” by twice that distance
and needs to be corrected. Flip the string over itself to check both
planes. Fix any problems now.

An “out of plumb” condition is one of the most common reasons door
units leak air and water.

*** End Stolen Text ***

It seems to me that the same check should be done for windows, but I
never saw this mentioned in any window installation instructions. I
know that besides checking the opening we should also check the window/
door itself before (and after) cranking down on the mounting screws,
but I've never seen this "Same Plane" check mentioned before.

Thoughts?