I think it's better to plan out but having no plan can make for a very interesting story at times x) For longer stories, I usually plan more haha. But for the one I'm writing, I barely planned at all and I think it turned out pretty well. I sorta plan while I write
I've always been the "wing it and see where it takes you" type, buuuuut that's often gotten me into a corner that I have no idea how to get out of without rewriting the whole damn thing. >.< So I've been trying to plan out at least the major plot points beforehand with anything I write now. It still gives you a good amount of room to make it up as you go, but you at least know it's going to get somewhere and that it will eventually end.
thanking up a story always beging with the first chaptor. From there you branch out and see were your story takes you and were you want it to lead. But first you must write a draft, place your thoughts unedited to paper and when you are done you then make changes that you find suitable not only to you but the story as well.
I don't think it's necessary to plan out the whole thing, but you do need to have a good clear idea of the end. Otherwise you may go off on a tangent that renders the beginning you've worked so hard on useless. Know the ending and keep it in sight.
When I was younger I got a Jacqueline Wilson My First Novel kit, they were good kits and I've had a few of them throughout my life while they still made them. One thing they came with was writing tips from Jacqueline Wilson. One of the tips was not to plan your story out too much as that's 'boring' and it's 'fun to surprise yourself as you write'. I remembered this tip so one day I decided to write a 'proper novel' inside a notebook I had (I'd used the notebook that came with the kit for a different story that wasn't really planned either but I'm not using it as the example because at the time I didn't think these stories were stories I thought they were films) and I didn't plan at all. I came up with the main characters' names and that the girl telling the story is naughty and that the girls had all been best friends once but then one of the girls moved away and the story was about them getting reunited, and that's it. Sure, I surprised myself as I wrote and I had fun writing, but I have fun writing AND planning my Lydia and Dynamite stories and the novel I came up with just writing willy nilly was shit and is shit to this day. Because I had fun writing it I'm going to go back, edit it, plan it and do the story justice.
In answer to your question I think planning is important and you should always plan your story, but you don't have to dot every i and cross every t so to speak. Plan, but don't over-plan and don't under-plan either and don't be afraid of going back and changing things later on.