Most of what I do now is paid programming, but when I used to cut for air I would make the breaks the correct length with black in there.
IE: If it was a 2 min commercial break I would leave 2 min and 2 sec of black.
The best thing to do is to contact the station/network that it is going to air on and ask for their Qulaity Assurance specs.
This will give you everything that they require so there are no surprises later.
Steve Cohen
Editor
O2 Media Inc.