I've looked at some Chinese books in Google Books, and I'd say the grid typesetting is not strict (when in horizontal mode), I've seem some lines compressed or expanded apparently to avoid a punctuation af the start of a line. When there is latin-alphabet text mixed with the Chinese, it seems it's enforced to fit the grid, but I've also seen some punctuation "half a box" wide (like the « and » marks).
This is from books published before 1980, like
this one (take the third from last line in page 154).