Not usually necessary and if the paint is not loose or flaking and shows no signs of bubbling or lifting then you should be ok to tile on top of it. If you are not sure see the paint stripping project for an incredibly easy way to lose the paint.
If you want to have them in any kind of symetrical pattern then design one on paper before laying. If you want a random finish, put them all on a table, close your eyes, mix them up and put them in a pile. Open your eyes and lay them as they are piled. Truly random is very often the best effect.