Titanium frame: using copper grease
I would say yes, use copper grease / ti prep / anti-seize on any titanium-to-metal contact (and especially ti to aluminum) that you anticipate staying put for a long period of time.

Length of the seatpost - yes
Seat post clamp - yes
Bottle cages bolts - yes
Rear derailleur - yes

The front derailleur clamp seems more questionable because it isn't threaded or a tight fit in theory, but sure, why not? Applying liberally is better— you want the copper grease to stay put through washes and rain to do its job.