Good idea, saves money. I cut my own hair, mind you Im going bit thin on top so I just shave it all to a grade one, I also cut my son's hair but he's only 3 and wants to look like daddy so its a grade three (at the moment because its winter) and my wife cuts her own hair but she's actually pretty good at it and changes her style and things.
General rule of thumb is 'if you ain't happy loosing it, don't mess with it' basically if you screw it up are you happy to shave it off LOL. Alternativly make sure you also have some cash and a hat, if you mess it up
1. Put on hat
2. Go to the nearest hairdressers and get them to sort it out (have a good excuse like you fell asleep on the sofa and little brother or sister had some scissors etc, to save face LOL)