No rules, except for the people who appoint themselves the rulers.
My guide: whatever
Rules I don't follow:
1. stainless spokes. or old red Bontrager spokes. or old white Bontrager spokes. or old Rolf black spokes. You get it.
2. Lugs. Even alloy lugs on carbon. Even carbon lugs on carbon (Colnago C40). Or lugless, like TIG, aluminum, Kestrel, Aegis, Look, etc. You get it.
3. Steel helps. Except modern steel.
4. Aluminum (50/50). Alan? Centurion Facet? Primus Mootry
5. Titanium. Is the Titan not C&V?
Rules I do follow, sometimes.
1. If 99.9% of your riding partners say "nice old bike," and really have no idea what it is.
2. If a hipster looks at it, grins at you, or looks over at another hipster and they both grin.
3. If it's an old steel bike and the owner has named it. I don't get it but I sure don't argue.
4. If a pretty girl says "pretty bike."
5. The ability to rust.