Sometimes I'm responsible for maintenance of an application that includes such elements as
    pack [button .b1 -text Lion -width 10] [button .b2 -text Hippopotamus -width 10]
[[show picture here]]  Notice that there's a problem with this fragment; 'Hippopotamus' is 
truncated.  That's not a good thing.  Most immediately, it's because .b2 has been assigned a
width that's too small.  Generally in Tk it's OK to allow things to have default sizes; there are
a couple of legitimate reasons to assign manifest sizes, though, including [[... and ...--but 
people also mistakenly set sizes when ...]].

'''[PWQ]''' ''''20 Dec 03'''' Can you give a real example of when the size of a widget (such as a label) need to be specifed. Without one my
comment is that this is completely unnecessary. Either you want the label to be shown in full or not. If not then the label text should be truncated before displaying. (ie Hippo...)
I get annoyed when I see TCL apps that people have '''assummed''' the width based on the font they have and it truncates on my display. This is the sort of bullsh?t that you  get with VB applications.

If those widths are constant values, they need to be correct ones.  Too-small labels make bad
impressions on users.  Here's a way I ask an application to tell me about itself:  I [source] in
    proc walker {w procedure} {
      $procedure $w
      foreach child [winfo children $w] {
        walker $child $procedure
      }
    }


    proc check_width w {
      switch [winfo class $w] {
        Button -
        Label {
          set original_width [$w cget -width]
          set x_extent [winfo reqwidth $w]
          $w configure -width 0
          set needed_x_extent [winfo reqwidth $w]
          set result [expr $needed_x_extent > $x_extent]
          $w configure -width $original_width
          return $result
        }
        default {
          return 0
        }
      }
    }
then execute
    proc c w {
      if [check_width $w] {
        puts "Widget '$w' isn't wide enough."
      }
    }

    walker . c

The truth is, that's not '''exactly''' what I do, but it's close enough.  For sufficiently
weathered applications, and especially ones that have moved around between
platforms, I usually pick up a few widgets that I'd rather find this way than by
having a customer complain.