I would agree with the views that the safest bet is to make sure you look neat and tidy and if that means getting a hair cut then do so!
Don't get fooled into thinking because its fashionable on the streets that it will be acceptable at flight school/airline interviews.
When I was doing my flight school selection one chap on the selection had quite alot of hair, styled quite 'free style' shall we say.
He didn't make the grade, this was partly to do with his scores on the tests but he also told me that the interview panel did mention his hair and told him that it would have been wise to cut it. They obviously did not approve of it and to be honest I would have agreed with them.
At the end of the day you want to give yourself the best possible chance and getting picked up on your hair in a negative sense is a real shame! Its just not worth it... remember they are looking for the 'right kind of people' meeting the grade academically is only one part of it, how you look on the day is absolutely vital. Remember first impressions count alot.
Look the part for the selection days (clothes, shoes, hair.. everything!) then once you pass and are in the flight school or airline then you can have the hair you want (to a certain extent)... after all the initial selection nobody really cares, its more of a cabin crew issue then for the pilots because they deal with the passengers face to face more. Having said that, it does not mean we can all go wild but flight crew get more lee way.
Good Luck!
Aka