A Question for Painters

Here's a question for the painters out there.

I have always composed in rectangles and was warned, years ago, against square compostitions. I notice, however, that the square is a very common format in the oil painting world. Do any of you have any pointers for using the square format? I have always used divisions of thirds in the rectangle format to provide general guidelines for emphasis placement. Any thoughts? Drop me a note or comment. I'll be interested in hearing. Thanks!