IMHO, double-tap modifiers are rare and should be kept for system wide shortcuts.

I use:

2 x cmd for LaunchBar activation

2 x opt for LaunchBar Instant Send

2 x fn to activate iTerm

2 x ctrl to open a Finder's window

2 x shift to activate TextMate

Fortunately for me, I use double shift for TM so I can deactivate it when TM is at the front, but I still think that if you want to use double-tap modifiers, you should provide a way to deactivate it and use a normal shortcut instead.