There's no line, no list somewhere, but there are bikes that you part and ones you don't. let's see if we can get a feel for this. of course these are my feelings and are not binding upon anyone, but this may help.
1965 Schwinn middleweight in 7 condition...part it our if you like, they made so many that there are still tons more in 9-10 condition that can be found.
1937 Roadmaster Supreme in 4 condition...this one you can't part out, there simply aren't enough of them out there in original paint in any condition.
Teens Sears Cheif in any condition...there are only a very few of these out there. as I understand it, maybe fewer than half a dozen.
I understand that some of the parts are common to other bikes, but this one has the parts that destinguish it from other Davis bikes and though you could simply buy those parts for another Davis bike, somehow it seems a shame to part out an original to make some sort of morphodyte cobbled together bike when as you readily admit you're not making a killing on the parts, and it could have been sold whole for around the same money, and kept together. I'm not against parting out some bikes, let's just not part out the ones that shouldn't be parted out, that's all I'm getting at. some pretty simple common sense could be used in most cases. if there are only a few of the bike you have, please keep it together, if there are millions go ahead. if it's in between, weigh it out and ask yourself, "which would be better, parting this out, or keeping it whole?" "how many are out there and how nice is the one you have?" "are you parting one of the nicest originals of what you have?" "could you sell yours to someone trying to gather parts for one and let him or her part out the one they have been struggling to gather parts for that's basically pieced together junk?"
this kind of thing never makes sense to me! some time ago someone parted out a freshly and correctly restored 1936-37 Hawthorne Zep. I watched as people gobbled up the pieces they needeed for their cobbled together junk, and no one thought "why don't I just buy everything, and then sell my pieces. would have saved them a bunch of money and time. instead they now have a perfectly restored rack, or tank that they are just going to have repainted when they paint their bike. it doesn't make sense.