Witnessed a scenario where an isv solution had modified an existing sys enum to add more values for its use and new elements value was only increased by +1. This is a bad design practice and can cause issues further down the road when its time for an upgrade and Microsoft adds new elments to that enum so you do not want to be in that situation. Therefore the best practice is to add your element with a value that starts with +100.
So for example i have this ABC enum and its C element’s Enum value is 3:
If my customization requires me to add values to this I should start at 103:
This will prevent any future upgrade conflicts that can be a nightmare for someone doing an upgrade.