I think he's looking for something similar to FASA's crew efficiency rating - the average skill level of the vessel in question.
While I can see the argument in favour of Owen's "they don't get one", it doesn't allow for variations in ship quality. Although it could be argued that that only applies to important ships!
I think I'd rate a ship as one of the following:
"green" (-2) - a training vessel or a new one the crew are not yet familiar with. This rating would usually become average by the end of their first serious mission.
"average" (+0) - the vast majority of vessels. This crew knows each other and their vessel well enough to function, but no more.
"expert" (+2) - they've been around a while and know their vessel well. This category would apply after a few years of uninterrupted missions. Any of the Enterprises would count here (given the amount of crew-shuffling they go through).
"veteran" (+5) - very experienced and extremely rare. These crews have been through hell together a number of times and function like a well-oiled machine - with no dissent and no hesitation. This might apply to a crack Klingon crew or the Voyager (which saw little or no crew changes for 7 years).
Apply the numbers given to your skill rolls.
This is all very hazy, so you might need to vary the mods somewhat in practice! And a veteran crew is significant enough they're worth detailing.
I have a sneak suspicion we might see something like this in the Starships book when it appears.
Jon
"There are worlds out there where the sky is burning, where the sea is asleep and the rivers dream; people made of smoke and cities made of song.
Somewhere there's danger, somewhere there's injustice, and somewhere else the tea is getting cold. Come on, Ace, we've got work to do."
THE DOCTOR, "Survival" (Doctor Who)