I know this seems a silly topic, but bear with me:

Any time Michael, Vito, and Vincent are "in business", their hair is slicked straight back. It's a signifier of their status.

For example, compare how Michael wears his hair at the beginning of the first film, parted, and then compare it to the end - slicked back.

Compare how Vito wears his hair at the start, compared to the end - from slicked back, to longish and unkempt. One signifies his power and regal status as Don, the other is a rustic gardener who has retired.

Michael in GF II is seen with his hair not slicked straight back only twice: During the assassination attempt and after, when he comforts Kay, and when he entrusts Tom with the protection of his wife, his children and the Family as a whole. In these scenes, Michael is not the Don. The other scene is in the flashback to 1941, when Michael is a civilian utterly disconnected to his Family's business.

Vincent, likewise, goes from wearing a more loose style in the beginning of the film, to adopting Michael's GF II slicked back style when he becomes Don.

Another hair signifier:

When Vito is an aspiring gangster (but otherwise, really, a nobody) he is cleanshaven.

After he murders Fannucci and has pretty much taken over the neighborhood and established himself as the local Don, he has grown a mustache (much like Fanucci himself, and Don Ciccio). It is a symbol that he has become THE MAN.

In GF II, when Fredo has aspirations of having something "on his own" we see he has grown a similar yet pathethic version of Vito's mustache in a likely conscious attempt to emulate his father.

To me, this signifies that the characters' hair was meant to signify certain things about the characters.