It's allowed but implementation-defined, so ebuilds relying on it aren't portable. PMS says: "doexe Installs the given files into the directory specified by the most recent exeinto call. If exeinto has not yet been called, behaviour is undefined. Files are installed by default with file mode 0755, or with the install options set by the most recent exeopts call. Failure behaviour is EAPI dependent as per section 12.3.1."
Not sure if this fits better into pkgcheck (or do both) or what.