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I impose no rules on myself. If the developers provide me with the tools (i.e. Grab Title) I intend to use them!:D;)
 
One rule I have that no one has mentioned is that I only use men for chancellor, steward, and spymaster if I have them available. It seems more in keeping with the era.

That's not a bad one, though I'd probably still allow women to take the spymaster role if she was the best. I doubt the role would be officially announced to your court after-all ;).
 
I try to limit my overall titles, depending on the realm I'm playing.

Like, if I'm the Byzantine Emperor. Would the Emperors call themselves "Kings of Bulgaria"? I doubt it heavily. But if I was, say, the King of Serbia, then being the King of Serbs and Bulgars makes sense.

The biggest kingdom union of the Middle Ages I can think of was the Germany/Burgundy/Italy HRE and that was barely held together at the best of times, so on top of the above I try to never have more than two king/duke titles no matter who I am (again, excepting GIB).
 
CK House rules for me...

- cannnot excommunicate dukes / kings
- can only claim province titles, no duke / king titles
- can only usurp duke / king titles
- cannot force-vassalise people outside my realm (unless they are a rebel, or I have a claim on their title, or I have king / duke title over their land, or if they are immediate neighbour)
- must force them to recognise my titles if I win (no more prestige exploit)
- marry within culture or neighbouring culture
- cannot force-vassalise heathens, only annex
- cannot recognise titles of force-vassalised rebels (no more -BB exploit)
- roleplay by trait (eg if I am Selfish, choose option that gives or is most likely to give Selfish trait)
- expand into neighbouring provinces only (or reasonably accessable by sea)
- dukes may only have one duke title (no "super-duke" WC exploit)
- may only revoke title of counts and dukes who own titles that they reasonably shouldn't (e.g. Duke of Leinster owning province of Saracusa, I can revoke his ownership of Saracusa)
- titles will be handed out to same-culture courtiers / vassals
- it is acceptable to F12-die courtiers that I don't want (they are "exiled" by my command)
- cannot start wars or assassinate if ruler has Honest, Just, Merciful, Forgiving traits (however if he has more "evil" traits then this can be exempted)
- males can only have court titles (except for spymaster, who can be female :) )
- play by Iron-man rules (cannot revert to a previous save if things go wrong!)
 
Opportunism!

I'll watch my reputation though
 
As a lower tiered vassal, it is perfectly acceptable to marry up, but once I am a King, I try to marry both male and female into other Royal Houses. If that is not available, then an important vassal or attempting to securing a nice inheritance, however, they must be of *proper* birth to marry into the Royal Family.;)
 
I try to maintain a better-than-tarnished reputation, and place counties under their default dukes whenever possible (which means I tend to have to take over the county and gift it to the duke).

Also only acquire territories that are "adjacent", although that includes being "sharing the same sea".

I also do not tolerate "holes" in my territory, particularly if territories are lost due to inheritance rules to another nation.