If your browser supports it, you could make a script or style-sheet that could check/uncheck whatever boxes you want. Opera has user-defined scripts and CSS built-in and FF has script plugins (GreaseMonkey, for one).
I’m an active member in seven or eight forums and this is the only one that doesn’t have such a setting in the “profile settings”.
I see no reason why it couldn’t work exactly like the “Attach Signature” to enable it globally in all posts via the user settings. So it feels more like it’s a choice not to have it, maybe it reduces traffic?
I guess I can just try and remember to click that every time.