There's a lot brushes out there, flat, angle, fan, wash, etc. I like to keep it simple and branch out as necessary. I mostly use my rounds and liners for the dirty work and a wash if I'm needing more coverage.
I find that if I have the right tool for the job, I can relax and enjoy painting more than when I'm mumbling obscenities trying to get my paint to do what I want with the wrong brush. Also, I like to replace my brushes every few months, when they quit holding water well or start to fray, spend a few bucks and get a fresh one, you're never sad to be using a new brush.