simple question - temporarily disable autosave?

[Mark P]

My emacs client normally auto-saves on a regular basis.  Here are the 
relevant bits from my .emacs that I'm sure I got from somewhere else:

;; auto-save stuff
(setq auto-save-interval 100)
(setq auto-save-timeout nil)
(setq auto-save-list-file-prefix ".saves/")
;; backup version stuff
(setq version-control t)
(setq kept-old-versions 3)
(setq kept-new-versions 5)
(setq delete-old-versions t)

The problem is when I'm in a read-only directory looking at a file and 
emacs flashes repeated warning that it's unable to auto-save.  I assume 
I can M-x auto-save-mode to turn this off as needed, but is there any 
way to make it automatically do this when I don't have write permission? 
 Better still, if I don't have write permission I'm not going to be 
modifying the file; maybe it can wait till I've made a change to the 
file before trying to auto-save?

Thanks for your help!