Man Bun in IB?

Coming from someone who had long hair for years during/prior to college, echoing/synthesizing some of the comments above - no man bun, and I'd suggest thinking about cutting your hair for when you start.  When you're more senior and/or have established a reputation as being very, very good at something, it's closer to 'anything goes' (still subject to institution-specific variation) but it's an unfortunate reality at the junior level that there are going to be a lot of opportunities for non-substantive snap judgments from narrow-minded people and that can include comments or judgment about appearance, exacerbated by a bias toward conservatism.  I would put a twist on one of the comments above, which says that 'getting it' means understanding that inconsequential-seeming things 'ARE the most important' - replace 'ARE the most important' with 'CAN BE unexpectedly important TO OTHERS whose opinions you may either want to or be forced to care about.'  At the junior level, because you don't have as much power, you are kind of involuntarily forced to have to care more about others' opinions on what I'll call non-substantive matters.  Now, in a service business like banking or law, even at the senior level, you may care very much about sucking up to or at least being non-offensive to a client who may have particular non-substantive dislikes... that's a longer term decision on how you want to live your life.  

One way I've thought about this issue in the past is, what's the upside?  I would suggest to you that for the first couple of years of your career while you learn the industry and get your feet under you, there's only potential net downside of keeping very long hair - you work for some grouchy senior with a bias against long hair, downside; on the upside, what, you're going to connect with someone who thinks your hair is cool?  I would suggest that the Venn diagram between people who think your hair is cool and people with similar interests or outlook on life is going to be very near to a circle and you're going to find those people and make connections regardless (e.g., if you originally grew your hair out for some reason other than pure aesthetics, say because you were into X genre of music or a ski bum... you'll still find ways to connect with people with similar interests sans hair - speaking from experience); related point, just because you chop the locks, your personality isn't going to change.  

Also, your mileage may vary, but I've found that sometimes 'fitting in' can provide unexpected fringe benefits - e.g., as a white male who doesn't wear anything too outrageous on the weekdays, the number of times others have felt comfortable making sexist or racist comments to me where they clearly thought we were in some sort of implicit confidence, has allowed me to make more informed decisions about things such as whether to change firms, whether to hire a particular team from another firm once I was in a position to hire external advisors, or give a negative reference; as I've gotten more senior, I've been able to more directly call bad behavior out and nip it in the bud, but even if doing so may feel harder when you're very junior, you can at least pay attention and get a lay of the land on personalities - it's a very small world.  Your first few years are all about learning, and you can learn all sorts of random stuff by remaining relatively close to center on some basic fronts like appearance.

Final practical consideration - I don't know if you have a daily hair care routine greater than 5 minutes, but I would suggest that when you're working around the clock, the last thing you want to add is more complexity to your morning routine after an all-nighter.