I buy 1)first when I see a good sale
2) when a fabric just absolutely calls to me, ie. soft blue/gray colors
3) when I need additional fabrics to make a project
I find that when I buy colors that really appeal to me, they usually find a home in my projects.
Hit a end of the bolt sale today; 40% off is you finished all the fabric on the bolt. 8 1/2 yds for $56 inc. tax, 5 yards of which were a gorgeous blue gray batik. The other was a green marble which will go in any Christmas quilts. It's hard to find good greens.