Definitely, as I go along. No contest. First of all, for me accomplishing any task is a more cohesive, thorough process if I can do it... AS I do it. Trying to make a detailed plan before I start a) is mind-numbingly boring, b) never is sufficient (the plan always leaves things out or fails to take them into consideration) and most importantly c) takes focus away from the task at hand and puts it onto following and/or maintaining the plan. Now this doesn't mean that I just jump into things with absolutely no consideration about what I'm doing and what I need to succeed, especially for complex things, but less is almost always better (and let's face it... most stuff isn't that complex).
I have a *horrible* time at work with this. I'm "required" to make a "detailed design plan" before I start anything... no matter how minor. Solution? Just do the job, *then* write the "plan" when I'm done, telling what I did. Sure, I could probably get in trouble for it... but anything else is just a waste of time and asking to miss stuff. Solutions, possible problems, and ways to address things come out naturally as I'm doing things - but NOT when I'm focusing on paperwork. Just the way it is. Wish me luck in getting the boss to believe it, though.
Note: I consider "plans" very different from "goals" - knowing what you're trying to do (goals) is important, but is usually a simple and pretty vague outcome-based thing. Saying how you will go about achieving it before you even get started (plans) is what I usually find counterproductive.