If it does crack don't despair as it may well be fixable, and those beauts are worth fighting to keep.

When that happened with mine, I got my CA (two kinds: really runny and the medium thickness 'normal' CA that behaves like your normal superglue. Use the runny stuff to seal the cracked surface. It may also run in between the brush handle and the knot depending how it was glued in. Take care that the CA doesn't run up into the hair via capillary action as that can completely ruin the knot. In other words, use sparingly! Once that has cured, use the thicker stuff to build up like filler in the crack itself. If it cracks wide, cut a fine sliver of the same wood, and CA that in place. More thinner layers is probably better than fewer thick ones. Keep doing that until the crack is filled, sand it flush, and you might end up with a very inconspicuous repair.

So if it happens, it's well worth a shot to save. But here's hoping that's a shot you don't have to take.