To each their own. If you like your hair long, then keep it long but keep yourself groomed well.
On the other hand, there is a lot of subliminal psychology at work in offices. It may be obvious to a lot of people, but others have have no clue about it. When I was in college, I had a professor in the business school that was very adamant about the notion that you should make yourself look and behave like the people in the positions that you want to be in. Basically, you need to be able to act the part. If you want to end up with the title of manager, director, VP, etc. and all of those people have short hair, and wear a nicely pressed shirt and a tie every day, then you should do the same.