Just thinking.... I know there are no "rules", but would you sash batik squares with more batik or go with "regular" fabric?

I would use whatever fabric that I thought looked best, unless your intention is to have an all batik quilt. I mix batiks with other fabrics and it has worked for me. :D

I think that more batiks will give you a blendy look. Other fabrics may give more contrast and make the batic squares stand out more. Audition the squares alongside assorted fabrics and you will most likely find a favorite.

Either way. I use both all the time.

I prefer a basic black or white to let the eyes rest and contrast the batik patterns. Some all-batik quilts can be too busy, especially if there aren't a lot of value shifts.

I like to use regular solids for sashing. It seems make the batiks stand out.