Personally, I think they should be on common template.
Or, in cases where there are many variants, on several multi-variant templates (fighters, night-fighters, attack, etc. etc. - depending on needs).
Separate templates for each variant only multiplies files and increases net transfer. With lesser number of multi-plane files You see "everything at once" (or rather: big part of everything).
Myself I use separate templates for different variants only on really big planes, or when the type was done by someone else and divide by variants is already well-established in the archive (Spitfire, Mustang, for example). Otherwise I use separate templates on "by country" basis, and mostly when I made several paint schemes from that country. If I have one paint scheme each per country I tend to merge them on single template too.