I have really long hair (I mean, it's different because I'm a girl) but my biggest concern is washing vomit and blood out of it. It's (obviously) much easier to do with short hair. Haha. Just make sure it's clear before you get targeted with chunks.
I also have long hair, and I keep it in a bun at all times on duty. If I just do a ponytail, it's still long, and no way I'm letting it fall into nasty stuff. Most of my female coworkers with long hair do the same. I've even seen guys who do. No vomit in my hair yet, except for one episode where I got completely covered anyways. Highly recommended.