Sorry I took a long time to replay, I had to do actual research for this one. I am super sorry if this is super long.
From what I actually gathered Rogue's white streak has a bit of interesting continuity due to artists creating new looks for her. Originally when she was part of the Brotherhood. Rogue had 2 streaks of white hair on the side.
As shown here in her first appearance.
http://media.tumblr.com/a9676eec85ab...o3B1r24r6r.jpg
Later on after joining the X-Men her hairstyle changed from two streaks, into a single one. This was in Uncanny X-Men #172
http://1.bp.blogspot.com/-Uby-vYLOe5...+172+Rogue.jpg
No explanation is given as to how she changed her hair. it just happened like Storm going channeling her Mr.T. At least that was given a reason why.
Later on the way her hair streak was drawn changed as she started sporting the all famous skunk stripe that we all love.
You see the beginnings of it here.
http://1.bp.blogspot.com/-jmSbMvr8I4...0/Fastball.jpg
and finally here...
https://c1.staticflickr.com/1/81/242...afb5c4c9b3.jpg
Then starting the late 90's her white streak began to fade away and only appeared in her bangs as shown here
http://ctrla.lt/comiclash/images/The..._ComiClash.jpg
After this you started getting the 2 white streaks on her forehead.
http://www.uncannyxmen.net/sites/def...ht/rogue46.jpg
From what I gathered one or two writers make a mention of Rogue's continuity changing white streak(s). In Rogues third solo series #10 the writer gives an explanation that rogue dyed her hair and adding the white streaks in order for her to look older which explains a lot considering retcon flashbacks to Rogues childhood show the famous rendition of the white streak during the 90's.
I hope this clarified the changes made to Rogue's hair from creative point of view to in canon. So... Do you have anymore questions?