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.
If titanium = nice frame then it’s worth doing things properly all the more. Yes, check with the maufacturer but I would think if the ferrules fit, then that’s deliberate and indicates you should use ferrules.
Then do a good quality job of installing the inner and outer cables, the attention to detail will add up to give the best braking performance.
Stainless steel (& slick) inner cables aren’t too expensive and should mitigate rust/corrosion.