I've used a can of spray after it was several years old and it worked just fine.
I love the idea of a used shower curtain to catch the overspray. I just tuck cardboard or old newspapers under it when spraying.
I too pin around the edges. I only spray baste on smaller projects. For the larger projects, double sized quilts and larger, I tend to stick with pins. The spray works great, I just would rather spend my money buying fabric instead of spray baste. ;)