ODE, Bullet and ubODE all have their quirks and "shortcuts" but the principles for a truly well made and efficient physics model are the same for all of them.
What physics does
First we need to think of what the purpose of physics really is. The physics model:
- defines "exclusion zones" where avatars (and physical objects) are not allowed
- defines walkable surface
- defines (to some degree) surfaces it's possible to rez on
Most decorative objects (such as furniture) only reall needs a simple "bounding box" style physics model to keep people from walking right through them. For such items, make a dae file with a simple cube, save and reuse whenever needed. Size doesn't matter, the physics model will automatically be scaled to the same overall dimensions as the visual one.
This method works for phantom objects too of course.
More advanced physics
Keep the overall dimensions the same as the visual model, sometimes we have to add one or more extra tris just to define the outer limits of the size for this.
Apart from that, add surfaces wherever you want an avatar to be able to walk and wherever you want to block an avatar from passing through. Nothing more and nothing less. For vertical, blocking, surfaces, you don't need exact placement, a few cm leeway makes no noticeable difference. But for horizontal walkable surfaces you want pinpoint precision. And as I alredy said, don't include lumps and bumps too small to affect the avatar in the physics model.