How about both.
If I have an idea already, then hey I use it then roll and fit the rolls into my concept somehow. If I don't have an idie and am having trouble coming up with one, I roll and then hope that will spark an idea.
Then again I have been runing point buys so it would likely always be concept 1st then spend the points.
By the way voted roll 1st since I've beocme idea starved of late, and instead of re-hashing an old idea I'd likely roll in the hopes of a spark of a new idea coming.