If you have encouraged this, then you should provide them with a way to generate stats that fits the image they've created. Nothing would stink more than to have the GM lead yo to this cool concept you'd love to play, only to have it dashed because you don't have the stats.
That is an important point.
How about, once they have their character concept, they arrange their attributes according to what they expect to be highest-medium-lowest (based on concept).
From there, highest attribute is 15+1d3, lowest is 6+1d3, middle is 8+1d3, etc (find some numbers and dice spreads that work for you).