It boils down to the architectural design needed to support the load. A wall dividing bedrooms, for example, may run perpendicular to the joists, and since it is there, it will end up provided some support, but it isn't necessarily required. It really boils down to the knowledge of span tables for various joists and trusses to see how much span they will need to support the live and dead load of the house.
If you have a concern, enlist the aid of an architect with whom you can work with and refer your potential clients to him/her for a basic plan which shouldn't cost too much for a remodel. You might also get some job referrals back from the architect. Go visit with one or more.
The best example may be to look at an unfinished basement and compare it to the upstairs. The basement will have the minimum walls required (unless the builder put in extra based upon a particular floor plan) and then compare to the upstairs. Both sets of load bearing walls will generally be stacked on each other and any openings will have headers of course.
Like the rectangular ranch style house, it will probably have a load bearing wall running down the near the center. Taking a half at a time, there may be a couple bedrooms, bathrooms, walk-in closets, kitchen, and/or living room all separated by perpendicular walls all providing extra support but there is no particular requirement that they actually be of a certain size just to hold a load. And in basements which usually have a load bearing wall down the center, you can have a whole half of the house as one room.
A deck example, when you build a deck using 2by6's or 2by8's as joists spaced on certain centers(16" to 24"), you support those joists with beams spaced according to the span tables of the size and type of beam and load you are supporting. If the span tables say you can support a deck with beams spaced at, lets say, every 8' then that is all you need by architectural design. As you and I and others get into overkill when we build something and decide to space the support beams every 4' to make a more solid deck, the extra beams will end up bearing some load but aren't architecturally necessary pursuant to the span tables and could be taken back out.
Hope this helps. About everything I've been around usually has joists and trusses designed to support the load of a house without more than a center support but sometimes we use floor joists/trusses to hold the entire span alone when the floor plan calls for a room that goes across the center line of the house which also applies to trusses above cathedral ceilings.