For me, it really depends. I have a tendency to ramble on (particularly in bits with lots of drama/dialogue), so I don't usually worry about moving too fast. XD However, I've very often rushed into writing without a plan and I have to say that it really just depends on how you approach it. On the one hand, if you just rush in and try to create a whole story right off, you're probably going to fail. But just writing can be awesome for brainstorming, and there comes a point where you've planned enough and you need to just get it done. The approach I've been favoring lately involves constructing a bare-bones outline and then allowing room for expansion depending on how the characters and plot play out. My approach is pretty "loose" I guess. I feel like my characters need a lot of room, because the story is about them and needs to be driven by them. They surprise me quite often. XD

However, it's still important to plan ahead. If you just start writing with no idea where you're going and no end in sight, you're likelier to get discouraged and give up. Not to mention that your story can be vastly improved if you know how it's going to end, because that way your tone and foreshadowing will stay consistent. I've had that lesson beaten into me before.