| When I run quote themes, I prefer them to be funny. If they aren't, then I
think they need to be especially poignant or clever. Compared to conventional themes with several different theme answers, a quote theme gives the solver only one big thing to discover in the puzzle--the "Aha," as Will Shortz calls it. We want to be sure solvers aren't let down by that lone "Aha."
I believe that quotes shouldn't be altered to fit the grid, because even the slightest alteration makes them nonquotes. Thus, you need to find a quote that fits symmetrically without any tweaking. You can't change THERE'S A SUCKER BORN EVERY MINUTE to THERE ARE SUCKERS BORN EVERY MINUTE just to get a balanced grid out of it.
If the quote author is famous, it's a nice touch to work his/her name into the grid--also symmetrically, if possible, though that's not essential.
Last but not least, verify the exact quote in several reliable sources. It's startling how many inaccurate quotes there are on the Web.