IMTU, there's a fairly sharp distinction between "Spacer" society - which tends to coincide with Imperial society, and is reasonably uniform wherever you are - and "Blue-sky" cultures, which tend to be varied except in a certain sort of parochialism. But then, IMTU the background isn't quite as complex as with the OTU.
Something like this happens IMTU (quite close to OTU). On it, moslty Imperial Services (Navy, Marine, Scouts, etc.) keep Imperial culture, that is mostly that for them and the nobility, while each planet has its own culture.
That's not unlike what happened with Roman Legions, that exported the Roman culture wherever they go, but most regions kept their own culture outside the more romanized cities (and even there when the own culture was strong enough, as in Greece or Middle east).
This has lead Imperial Culture to have some distinguishing traits:
- Formality: as it is mostly the culture of some services (most of them highly militrized), it tends to be a very formal culture. Honor, respect to superiors and sense of duty are very hight on it.
- Lack of popular distinguisihing traits: for the very same reasons, it lacks most of we see as distinguishing traits for a culture (legends, holydays, music, and folklore in general). Of course, some of all that exists (mostly among the low ranks). Soldiers/enlisted crewman have their songs, their legends, etc., but not to the extent most cultures have
- Standarized: standarization is one of the basis of the Imperium. Every imperial rule/item is as standarized as possible (e.g. IDPs for ships). So, an IN crewmember can be rotated to any Imperial corner, and rules and hardware will be the same. Imperium keeps rotating units so that they don't attach too much to their Imperial region and they keep loyality to the Imperium at large, not to their region (something that happened to Roman Legions).
Imperium services are what keeps Imperium together, and efforts are made to keep them separated from local cultures to avoid fragmentation. They adapt to them, but they are always kept (to some extent) aside, and they try to enforce Imperial custoums among themselves.
As those same Services usually collaborate with local ones (e.g. the Marines on lease to local units), efforts are done to avoid such assignments to last too much, and are often given by people from other Imperial regions. By taking those steps, it is hoped that more Imperial culture will influence local one than vice versa, so helping to give (mostly to local services) some sense of Imperial, rather than allow Imperial services to be too attached to their region. The Imperium is quite paranoid in this sense.
Only nobility is not so rotated (for obvious reasons), but they use to be quite apart from their "subjects", and their command uses to be quite formal only, with planetary governments runing the day-to-day of the planets and the planet's nobles being some kind of ambassors/governors, with limited true power in the planet itself (some more in its Planetary Navy, should they have one, as Imperium thinks they must be kept some degree of control on them). While in some cases the local noble may also be the local governing person, this is rare (and shuned by the Imperium if possible), and in most cases the nobility keeps their affairs in meetings with noble counceils and keeping Imperial affairs, while moslty leaving the planets to themselves (as long as they don't go against Imperial interests).
Off course, all of this is IMTU...
