Most film/tv composers I know and work with including myself use video synced in an external host. This is for several reasons, but mainly it’s because as various cuts come in you only have to reload the film once instead of 30+ times or so for each cue.
I tried working with video internally in Cubase for a while, but it was a real pain when I wanted to slide the cue around by 2 frames a few times or change start tempos, etc. I have a mac mini server hosting the video in Logic (though you can use another version of Cubase, DP, Reaper, Pro Tools whatever) that locks up using MIDI over Lan and Midi Time Code. Works great. I also run Logic and Cubase simultaneously synced with the IAC MIDI driver on my MacBook Pro when traveling.
As far as project management, one project per cue is certainly the way I prefer. Multiple cues in a project can be unwieldy and dangerous if you’re not careful with time-changes, tempos, etc. It also highly encourages writing from the beginning of a film to the end which in my own experience is not the most conducive to theme development/creativity. I have a template created for a film that has the sounds/instruments I’ll be using. The template morphs and changes as the project goes on.
All that being said one of my biggest beefs with Cubase is that it is REALLY lacking in tools to make sharing of instruments/audio/midi/tracks/themes etc. easy between projects. DP definitely has this sewn up with their chunks and Pro Tools is a close second with their Import Session Data feature.
Cubase has the Export/Import Track Archive feature, but it is quite flawed on several accounts. The first of which is that whatever timecode your Track Archive was exported at, when you import it into your new project it will change the timecode of your current project to match. AND if it was earlier than the project being imported to, will shift any events currently there later. This is fairly maddening. There are some tricky workarounds to making this kind of work, but I would love for Steinberg to simply give us the options of what data to import from track archives similar to what Pro Tools does. Allowing us to import session data from an already existing project like DP or PT does would be even better as it would save the step of having to open up a previous project export the data in the first place!
Anyway, this has gotten long. Hope that helps.