I use the "lay them on the bed to see if I have enough blocks" method, also.
If you have a design wall/board, that is helpful, too.
If you take photos of the different layouts, and then look at them later, I've found that's helpful, also. I see things in the photo that I missed, before.
There are so many variables to the answer:
How big is the bed?
How much of a drop do you want?
Do you want it to go over or under your pillows?
How much fabric do you have available?
How do my blocks look the way they've been laid out?
Some settings seem to look better with an even number of blocks, some seem to look better with an odd number.