The nick will grow, as to how fast, I couldn't tell you. I would leave it on and check it often.
As for leaving the cover off, I wouldn't. I would just modify it to make it easier to remove. Alot of people have it left off, but I do know of one instance where the belt actually did get chipped from debris under the hood.
If the t-belt does break however, as long as you don't have bigger cams and any head modifications, it shouldn't do any damage seeing as how the zetec is non-interference.