Jessie T. Wolf
It Gets Even Better!!|
I agree it doesn't hurt to ask, expecially if the artist just draws for fun, or specificly says "I'm bored, give me something to draw!" But the key as you said is being prepaired for a No.
There's a big difference between "would you be interested in drawing X for free?" and "HEY, You should draw me X, and give it to me for free!!"
As with all things, politeness goes a long way :)