If you are going to waste time and money (?) on recording, you might as well be a little prepared. Some of the great songs that have ever came out my have been improvised, but as far as doing a whole thing, it might not be so good.
Be prepared with a certain format, then you can tweak it later..depending on studio time. I was in an original band that would tape EVERYTHING in practice, and just pick out stuff if we liked it, and started praticing it with the new stuff in it.
Good luck.