The quality of brush you use when painting does make a difference, but they don’t have to be expensive.
I had a chance to go by the art store today and I picked up some very inexpensive brushes for painting in gouache. I immediately felt that I could get the accurate strokes that I wanted on the surface when using these new brushes. The best thing about it was that they were very cheap.
Here is a link to the Princeton Brushes I purchased, although their website doesn’t have some of the shapes that I picked up.