You can have brushes overlap each other without any problems at all, so disregard that 'advice'. The only downside of overlapping brushes is a slightly increased compile time, and less control over how and where brushes are split.
Basically, UsaKilleR's advice is an inverted advice, since it should be something like this:
BUILD ALONG THE LARGEST GRID-SIZE POSSIBLE
IN OTHER WORDS, DO NOT USE A SMALL GRID-SIZE WHEN CREATING A BIG BRUSH
THIS WILL KEEP YOUR MAP NEAT AND TIDY, WHILE ELIMINATING A SHITLOAD OF POTENTIAL ERRORS
(oh, and try to avoid overlapping brushes, for the sake of keeping your map neat and tidy)