I don't claim to have the "right" answer about this by any means but I think putting a title on your PS is not a good idea mostly because it is so hard to avoid titles that are incredibly lame.
Though I think "passion takes the reins", for example, is a really cute title, the fact that it is pun-esque and reminiscent of the title of a Saddle Club book might
be annoying to an adcom who has read hundreds of more or less identical PS's about "passion" for law.
Obviously it is your choice and it probably just depends on the mood of the person reading the essay. I never thought of titling my PS until I read Anne Ivey's book and she said not to do it.
ps liz I hope that wasn't offensive, I love your avatar