I have not done one yet, had one planned for a baby quilt but used a different pattern. The link I found said to use a number that is a factor of nine, like 3,6,9. I don't see where this exactly holds true as one of my favorites from this board was done with 4 fabrics. I say find the group of fabrics you like and the placement that appeals to you, and go for it! This is my favorite of all the ones I have seen so far.
http://www.quiltingboard.com/t-76729-1.htm