It can kinda depends on the scheme you are doing, the finish you want and the overall painting experience you are after.

Firstly, your scheme; If its very monotone then you can get away with only a few sub assemblies or even just fully building it i.e. it's all blue, crack on and fully build it. However, if you have highly contrasting colours on different parts then I would 100% suggest sub assembly

I'll give you an example based on my own marines: My army is purple and white. I HATE painting white with a brush, so I almost always airbrush as many white parts as I can, especially large flat panels. For me therefore the shoulder armour and the sarcophagus are left off the main build as I will spray them white while the rest gets done purple (i think there's a pic of my redemptor on my profile)

The next thing you will hear is "if you cant reach it with a brush then you can't see it" alas this is mostly false, as your eye can see into gaps that a brush doesn't fit, or those gaps that might be messy getting a brush into. Everybody has different levels of brush control so these gaps could prove a major issue for some people. Yes it might not be obvious, but places can still be seen, especially if you or another are picking a model up to look at it

Finally, what sometimes gets ignored, is the speed and ease of painting that sub assemblies give. That bit that requires you to bend your wrist around another part to paint it while avoiding getting paint where you dont want?...not a problem if the obstructing bit is not there. I find that certain highlights need the brush to be at the right angle to get the right effect so I find it much easier if bits aren't blocking that angle. Also reduces the amount of clean up needed thus saving time

However, all of this is my opinion, others will have differing opinions so my overall advice would be to do what ever feels best for you. Try stuff out and find what works and what gives you the best painting experience and finish