Aspiring to be a fair manager is an admirable goal - but it also a bad one.
I like to recall a story I heard from Seth Godin about a marketing company that used census data and created the "average man". They had a huge profile set up that came up with height, weight, appearance, schooling, etc. They then decided that it would be awesome to have a real person be marketed as the "average man". However, they couldn't find anyone that fit all the criteria. It appears that the average man does not really exist.
This story always pops into my head when people discuss being "fair". The reason is that fair is a construct that can't really apply to anyone. In order to be "fair", it means that the everyone will end up being unhappy with the majority of the rule.
In the end, the manager who tries to be fair will just make everyone unhappy. Not quite sure that should be the goal of the manager.
The answer isn't to be fair, but to be equal. A fair rule is one that is applied to everyone in the exact same way. An equal rule is one that has a meaning behind it and is applied to people as appropriate.
The best example I use is working hours. If employees are paid for working 8 hours, you want them to work 8 hours. However, unless it is necessary, it doesn't have to be the same 8 hours. William may want to start earlier and leave earlier because of child care issues. Tasha may not have kids, but would prefer to come in later and stay later.
Being fair means everyone has to work 8 to 5. Being equal lets people adjust the schedule to meet their individual needs without having a negative impact on productivity.
Fair is easy for a manager - apply the rule exactly the same. Equal is the right way - apply the intent of the rule.
Managers should do the right thing, not the easy thing.