I'm with you Sokoservices. This is yet another of those questions that's hard to answer. As long as I've been in this business, I've always heard the age-old adage that for every requirement in the NEC, there is an exception to that same requirement somewhere. Of course, the acronym "AHJ" is the true wild card that can literally shred your code book into packing material.
Technically, a residential panel today is "dead front" when you think about it if it's full of breakers. OK.......maybe the main lugs might be a questionable area. I can tell you that in my case, I would have to get into my GE panel with a coat hanger to get in trouble. Even with the cover off, all lugs are protected from accidental contact. If I wanted to touch the lugs of my main breaker, I would have to seriously grease my pinkie finger and force it into the hole where the hex wrench is inserted to tighten the lug.
I agree that it's not formally stated, but assumed to be "code" to have a panel cover in place. It's mostly about the manufacturer's approval. Perhaps their panel isn't approved or listed for use without the cover in place. You have to admit that this is the case with plenty of electrical equipment.
I can hang an 8', two light fluorescent strip without the covers. It will still work and there really won't be any additional live parts exposed. Still, this practice is wrong since the fixture assembly isn't complete per the approved listing without the covers in place.
Fear not: There are plenty of folks here who will jump in and try to help further with much better information than mine. Just give it a little bit of time. I learn something new here every day. You will too.
