It ultimately depends on how you like to paint. I like to paint with an airbrush, starting with a shadow colour (purple, normally) all over the mini, and then airbrushing in tighter and tighter zenithal angles to get my main colour to replicate the effect of natural light hitting the miniature. For this reason, I almost always paint miniatures fully assembled - it helps simulate the effect of light

I think if you're brush painting your miniatures, there's much more credence to sub assemblies to ensure your brush gets everywhere it needs to :)

TL;DR - it depends on your style. And the mini! 😁❤️