X. Symbolic
Not many people realize, but having long hair can be really dangerous. It can easily get stuck into tree branches, especially if you are a shinobi. An enemy can yank it during a deciding battle or it can just get generally in the way.
Yet Madara had decided to grow his hair long. This decision had ended up in that; his father did give Madara a talk about how impractical it would be, not to mention how girly it was. Unfortunately once Madara made up his mind about something he would have his way, even if it meant his father didn't like it. This was something that Uchiha Tajima brought up each passing day for the next month as Madara refused to cut his hair, it slowly starting to grow longer.
As each day passed, Madara's hair did grow and so did his powers, it being like a sign of his greatness for the future. Because if there was one thing Madara knew, anyone who had long hair and was a shinobi meant that they were more talented than anyone else. After all, it was a sign of surviving because hair could only grow when you were alive.
So, when Madara's brother Izuna died, for a moment he played with the idea of cutting his long hair for the symbolic reason of his mourning. But he soon casted it away, because there was still one more battle before he could do it, because he wanted to be in his most strongest state when he would face the Senju; only after then, when he won, could he be weak and cut his hair and mourn his brother.