Does {1em} mean {1em plus0pt minus0pt}?

Is plus0pt and minus0pt automatically added if I write, e.g.,

\setlength{\abovedisplayskip}{1em}
\setlength{\jot}{0pt}
\setlength{\parsep}{0pt}
...

-

Only if TeX expects a glue. Lengths with skip in the name are always glues (unless the programmer is a bit crazy). But there are some other length "variables". For example \hsize accepts a ⟨dimen⟩, for example \hsize 4in. There the plus and minus do not apply. Charles has shown the example of \hbox that accepts a ⟨dimen⟩ as well.
Semantically, yes, but not syntactically: \hbox to 10pt doesn't expand to \hbox to 10pt plus 0pt minus 0pt.
Well, \hbox doesn't expand at all. But note that you can use a skip register to specify a dimension. The plus and minus parts are simply ignored: `\skip0 2cm plus 5cm minus 2cm \hbox to\skip0{This works, but is overfull.}' – TH. Sep 23 '10 at 0:34