I'd say that for me, any ship up to maybe 1000dt needs a detailed floor plan. Above that, I'd just as soon have a "block diagram" that mentions engineering, bridge, weapons, living, cargo a fuel sections, but doesn't actually "drill down" to the details for all those sections.
For example, a 1000dt trader could have a "cutaway" view that showed where the decks were, but only floor plans of the bridge and staterooms areas, and maybe engineering. The rest are just big empty rooms or tanks full of fuel - only interesting for their overall shape.
Smaller ships, like "adventurer" class ships (up to 500 dt at the most) should have full floor plans, considering that they are likely to be used as settings, and they're still small enough to fit on one or two pages with reasonably sized squares.
I guess that narrows down my preference to a simple rule - if you can't fit the whole ship's deckplans on two pages with 1/4" squares, then don't map the whole ship. Give us a cutaway and maps of the "interesting" parts (bridge, engineering(maybe), staterooms, etc.). It's also ok to give a repeated "exemplar" deck like "cargo decks 1-4 look like this", as do the 2 fuel decks, or "passenger decks 1 and 2 follow this plan" type notations. If the GM needs to flesh out the entire ship, he can cut/paste all those parts together himself.
This goes double for large warships where whole decks are nothing but munitions or particle bays. It's fine to have the ship block diagram mention these decks as "particle bay deck - 48xParticle bays and 80 tons of sandcaster storage" or something like that. In these cases, a few words are worth a thousand pixels.
