This bug has been around a while, but I just discovered the workaround 
someone posted on the swiki.  The problem can be reproduced simply:
- in 3.6 image, open a new StackMorph
- add a TextMorph to the first page
- set text alignment to centered
- from the Stack's red "menu" halo button, select "copy & print" -> 
"print Postscript to file" and accept defaults.
- a walkback appears: "PostscriptCharacterScanner doesNotUnderstand: 
#setAlignment:?"

This problem can be fixed by the suggestion posted on the swiki's 
"Postscript support" page.  The fix is to modify the class 
'PostscriptCharacterScanner' to be a subclass of 'CharacterScanner' and 
remove from the former the instance variable 'font'.  I do not know if 
this fix causes other problems.

Here is the change to PostscriptCharacterScanner:

CharacterScanner subclass: #PostscriptCharacterScanner
	instanceVariableNames: 'canvas paragraph bounds curPos foregroundColor 
emphasis '
	classVariableNames: ''
	poolDictionaries: ''
	category: 'Morphic-Postscript Canvases'