there is no LAW that says you have to use foundation...the quilt police seldom check to see what you have really done :)
the foundations make it easier when using cuts that may have bias, keep your blocks stable....when crazy quilting especially the stability really helps if you are embellishing your blocks. If you use fancy fabrics some may stretch out of shape while doing your hand work, the foundation helps keep this from happening and with string quilts it is so much easier to sew the strips to a foundation and then square up, if you are just sewing the strips together the block can stretch out of shape and be wonky...does not necessarily mean its wrong. another consideration...foundation pieced quilts can be quite heavy...along with all the extra seams you have a whole nother layer of fabric. I love making both types of blocks, crazy and string,..can not imagine not using a foundation, but that is just me, know one says i HAVE TO.