Simple rule of thumb :
If you can't tell they have air in them, they don't /bcforum/images/%%GRAEMLIN_URL%%/smile.gif
IMO, the point of bench-bleeding the MC is to get the working cylinder area as full of brake fluid as possible; only to make bleeding on the car easier. With no fluid in there at all, a full stroke of the MC may not generate enough pressure to force fluid through the system, making bleeding difficult or impossible. If this turns out to be the case and the MC is already on the car, you can solve the problem by bleeding right at the MC outlet (with rags stuffed underneath to preserve your paint). Just loosen the fittings as someone depresses the pedal, then tighten them before releasing.
BTW, I disagree with both Geo and Ron about the symptoms of air in the system. The air will compress instantly and not change overnight; so if the pedal falls overnight it means there is a leak somewhere. (This is a classic symptom of a bad return valve in the MC.) And while being able to "pump the brakes up" may result from air, it also results from the normal travel in the calipers and cylinders (plus lots of other possible problems, including stuck pistons and poor adjustment).
If you can lock all 4 wheels with a single stroke of the pedal, then there is not enough air in the system to cause problems. And any tiny amount remaining will work it's way out as you drive.