So far, every quilt I have made was planned ahead of time. Only the width of the borders may get changed. Someday I need to try and whip up something that I make up as I go.

it depends if I am doing from a set pattern where I buy all the material or if I am using up scraps. If it is with scraps, I get the border fabric etc after it is done

I find a block or get an idea I like and fly by the seat of my pants and it usually turns out just fine, if not it's an opportunity for further design!

I'm going to say I do 50-50 when it comes to layouts. There are some that are drawn out and put in a project folder to be religously followed. Others like you said just sort of metamorphosis as the pieces are sew together. Changes to the project happens so often that I tell those for whom I am making a particular quilt that what they saw on paper has a 50-50 chance of being changed .

Sometimes i plan a quilt from the border fabric, sometimes i buy all the fabric for the quilt at one time. A few times, the fabric i picked for the border doesnt do the quilt justice and i have to use something else. Sometimes i dont have enough of the border fabric, so i will put something in the corners or piece the border. I've learned to be flexible- sometimes a better idea comes along than what my poor uncreative mind comes up with. :)

I do that too. Usually, I pick a focal fabric, and try to get a few others that go with it. Once I have my top ready, then I look for boarder and backing fabrics.

Many times I find a fabric I know I want to use as a border and plan the quilt for that. I do it different ways with each quilt, some I make the top first some I choose the border first. Several quilts I have chosen the quilting first. I don't see the point of making quilt choices in a certain order.