While ideally the filmed footage would be shown at full width, there are a few moments where the compositions are severely vertically cramped, implying that something else might be going on there.

It is possible that while much of the filmed footage was indeed widescreen safe, every once and a while they may have had to zoom in to filmed footage to account for missing sets, boom mics and other anomalies that might not have been noticed when shown in 4:3.

To be fair, most of this happens in the first season.

I do hope that a Blu-ray release of B5 would do something about the effects sequences and offered as an alternative, but I expect such an undertaking would be cost prohibitive without a Star Trek-like franchise juggernaut behind it.