Most of the time I'm a mid-grade on brushes. A round 0, as long as it holds a good tip is my go-to for cockpits, lasers, that sort of thing. That said, I do have a Kolinsky sable 0 reserved for extra-fine details, murals, etc. There is something to be said for paying for quality tools. For the most part, I've found that you tend to get what you pay for in brushes. If you're used to spending only a couple bucks a brush, do yourself a favour and go up to something in the $7-10 range. That will put you into better quality without completely torturing your wallet (real sable will set you back at least twice that).
I find that going smaller ends in frustration as the paint dries out on the brush before I get to actually applying it to anything. More bristles, more retained moisture, less paint drying out before I use it.