It depends on the browning. If it's just the skin being affected, then no worries - these spots are usually just oxidized chlorophylls that will disappear when you cut into them anyway. If there is any brown color inside, it might indicate bacterial contamination or bruising. To prevent either from happening, try to store your avocado at room temperature instead of in the fridge so that its flesh doesn't become too cold and stay firm before you want to use it.