I can confirm, that there is no rules.
From my near 30 year experience I know that each manufacturer of capacitors provides its own rules for marking if applicable.
Especially you can buy capacitor engineering kits are they are most often marked if they are expensive.
e.g. for ATP porcelain capacitors you can orcder laser making option (http://www.atceramics.com/UserFiles/100b.pdf
The same could be available for other suppliers.
Generally, small and cheap parts are not marked.
Aluminum, tantal, mika, foil are typoically marked but because they are marked without laser the package size must be big enough for it.