When a manufacturer isn't super specific about the orientation of the outer foil (like Auricap or V-cap, who use differently colored wires), then we do not presume consistent outer foil orientation.
There are threads on other forums where outer foils have been measured on various film caps and found to be inconsistent, so in the case of the Daytons in the Mainline, it seemed most appropriate to orient them the same way. When installing upgrade capacitors that do have the outer foil marked, then your assertion would be correct regarding where to locate the outer foil.
-PB