I've broken so therefore bought new about half a dozen props total so far. All the new (official, DJI) props I've bought have either been perfectly balanced or nearly perfectly balanced right out of the bag (I have a prop balancer)... so imo in general people don't have to really worry as long as they're buying official props. It's also easy to get cheap knockoff props if you aren't careful, many Amazon stores advertise official props and send knock-offs, so be careful!
I suppose you might also want to balance a prop if you happen to damage one due to a tipover, hitting a branch, or whatever... although imo if there's anything more than a minor scuff on a prop I say replace it with a new prop... ESPECIALLY if there's any kind of chip or actual pieces missing... like, what would you rather replace: a $10 prop or $1500 of Phantom, FPV and camera equipment if your damaged/broken prop decides to completely come apart during flight?
DattaGroover: a prop balancer like the one in that link works as thus: obviously you have to have the base of the balancer on a perfectly level surface... then you screw a rod into a propeller as shown in the pictures, and place the prop/rod onto the balancer horizontally, again as shown in the picture.
If the prop is perfectly balanced, it will basically stay horizontal. If one side of the prop is heavier, that side will "drop" and the prop on the rod will roll. You can either then shave weight off the heavy side OR (better option imo) add tiny bits of weight (I use tiny strips of electrical tape) to the light side until the prop is balanced so that when it's placed horizontal on the balancer, it stays horizontal.