Great idea - metric as well as imperial, of course. If there's any point in having the grid on the rectangular room option then polygonal rooms are also worth doing.
Yes, having the ability to choose a point of origin, or even a fixture to offset from would be smart.
What would be even better would be if the thing could actually create a visualisation in the Render mode either as an object or a texture. Currently the texture option doesn't allow you to show a ceiling grid texture that you can easily relate to the actual fixture locations, so the fittings usually look like they're unrelated to the ceiling grid.