i think you can use many background fabrics as long as they have a similar tone/value to them.
i have different background fabric in all of the 4 blocks i posted. if you click on download and zoom in you'll notice each one is different.
the overall theme is indigo but if you look each one has a slightly different pattern.
i have about 8 different japanese indigo patterns to choose from and i have about 30 blocks done - its not distracting at all that they vary from each other.
i guess you could say i'm doing a scrappy quilt except its all the same tone of red, indigo, and white that it doesn't really read to the viewer as scrappy.