Fixing permissions is recommended before and after installing any software to your mac and install software updates. There are various software available on the Internet to help you maintain your Mac maintained and in "good health". Software such as Onyx or Cocktail to do a great job with not only improve and verify the permissions for you but also cleans internet history, cache, temporary files and run the script optimization.
If you for some reason can not boot your system, before you do anything like reinstalling the OS, try pressing command + S at boot-up system and will make you into console mode. Type / sbin / fsck-fy and press enter. Do it several times until it says "System Modifications". Before you use "fsck" command, I advise you to open a terminal, type man fsck and read the man page carefully. Not all software errors are as easy to fix by using the above method. Some errors and rogue system can indicate a serious hardware failure, so if the problem persists, it is best to bring the Mac to an Apple authorized service for check-ups.