you have to know how you work.
I can do both (writing with an outline/without an outline) but have issues with both of them. It really depends on how you work, if you have a natural flow and can just sit down and write then go ahead and do so but outlining something can help make a story more concrete and you know where you want to go, you just need to decide in detail how you're going to get there.
I didn't used to outline, I still don't do it always--esp for short stories, but I have several outlines (very basic) of stories in my notebook and If I get an idea I can go back and add it here and there. I find this is useful to me because I have a really really poor memory and if I have an idea I need to write it down .