I'm pretty sure it's important to a quilter's sanity if she/he is flexible.
My quilts/sizes are planned ... sort of! However ... things don't always fall in with my original vision. When I got the idea, "I'll make a Grandmother's Fan quilt and use up these many yards of lace I've had for 20+ years", I ordered 8 'coordinated' colors of fabric of various lengths. A couple of weeks ago I washed, starched heavily and pressed it all. Not only did the combination of colors look hideous, but I only had enough lace for 18-1/2 fans instead of the 20 I wanted. I didn't find a pleasing layout for an 18-fan quilt, so am using 16 and only 3 of the fabrics ... it will be a lap quilt. I bought 2 more fabrics this past week for wide & narrow borders and I'm happy to say that they are perfection, so I did a fairly exuberant happy dance after UPS delivered them yesterday a.m.