I pretty much do improvisational designs -- like doodling with the machine, so I don't draw out a pattern before sitting down to free motion. I do however, think it through somewhat and have a general idea of how I plan to fill the blocks. I often will take a photo of the entire quilt on the design wall, as well as a close up of a single block. I print these out on my printer in black and white and I doodle on them, figuring out pleasing designs and looking for landmarks for myself. So I'll not that I will make a loop that extends from a specific point on the block, back to the center, for example. I often post those doodles above my machine as I work and use them for reference somewhat.
As for how much to do -- it takes some practice to get an even coverage, but one way to do it is to visualize say, a quarter on your quilt top, and always aim to have the stitching lines roughly the same distance as a quarter would be -- use whatever object fits with what you plan -- it could be a hands-breadth or a coffee mug, if you want looser coverage.
As to your question about the design mattering more to front or back. Not totally sure where you are going with that, but I always relate the quilting to the blocks on the front. What happens on the back is secondary and a nice plus, but I don't specifically quilt with the backing in mind. You could, if you wanted to. I've seen people do outline quilting of the background fabric, and then whatever happens on front is a happy accident. I think that works in some situations, but not as a generally useful technique.
RST