Is nesting grids a good idea?
It's better to use nested grids that are easy to read, than complex spanning.  I try to avoid spanning in most scenarios, unless it's just 1 simple span.  But when in doubt, nest the grids, because that way future layout changes won't break everything where as spanning is directly tied to the number of columns you have.

A great example of this is headers and footers, you wouldn't want them to not fill the width of an app,