dired-copy-preserve-time affects file modes, too.

[Roland Winkler]

I suggest that the docstring of dired-copy-preserve-time should
mention that the value of this variable affects also the file modes
of the copied files (cf. the optional arg keep-time of copy-file).

Furthermore, I believe that the docstring of the command
dired-do-copy could be more precise. Instead of saying

  This normally preserves the last-modified date when copying.

it could say something like

  Whether the last-modified date and file modes are preserved
  depends on the value of `dired-copy-preserve-time'.

Roland