High-side vs low-side reverse polarity protection
It depends on your application. 

The main issue with low-side protection is that you are disconnecting your ground reference. Many different systems work on the assumption that the 0V/Ground/Earth is shared between the devices. There can be many obvious and hidden ground connections.

If by way of example you have a circuit that is connected to ground by some other means - such as a USB device connected through shield to a PC which is in turn connected to earth and from earth back to your supply negative terminal. In this scenario, your low-side reverse polarity protection is effectively bypassed through this other current path.

If on the other hand you are using a battery connected only to your device, then there is no harm in doing low-side protection as there are no hidden ground paths that can bypass it.

Switching the high side on the other-hand is usually not an issue, as you would typically connect all the grounds together and have an individual power supply - it's unlikely there will be a hidden path from the power supply positive terminal through another device (*).



(*) not impossible - some systems, e.g. some cars, have positive earth, meaning the positive terminal of the supply is effectively the common terminal (car chassis).