Should internal brake cables use ferrules at the entry point to the frame? I recently got a titanium frame with internal cabling. The holes where the brake housing enters the frame are just big enough to fit a ferrule. Should I use ferrules or is it OK for the cable housing to go directly into the frame without them?

If the ferrule fits use it. It's neater that way and it protects the frame from the sharp end of the housing. Ferrules that have a seal against water ingress do exist. You should consider using these as they keep water (and rust) out of the housings. A glob of grease at the contact point between ferrule and frame eliminates rubbing noises.

Even if you cut and prepare the end of the housing properly the metal spiral retains sharpness. And anyway steel is harder than Ti and will let a pressure mark at the point of entrance. The ferrule puts a softer buffer between the contact points.