The fifth is balance. If a charge is present it must be either full size in the middle of the flag or small size at top hoist. If there are many small charges, they must be placed evenly, not to unbalance the design. No metal on metal, no smalt on smalt. As flags can adopt more colours then shields, its translate as no pale color near another pale color, no dark alongside dark.
Writing is a language, flag is another.
Writing is the last resource when a designer has not been able to represent correctly what he intends without words. A beautiful monogram on simple field can be a striking design.