Since the definition of <lg> is "contains a group of verse lines functioning as a formal unit" I think that we should require at least 2 <l> or 2 nested <lg>s.
i.e. inside a line group you are either having multiple lines, or you are nesting line-groups of multiple lines.
This is because I believe the definition requires that you are grouping multiple things. A single line inside a line-group is poor practice I think.