I have a desktop and a laptop, both running KDE.

I use KMail with a mix of disconnected IMAP (dimap) accounts, regular IMAP & POP accounts.

When I travel with the laptop, I want everything sync'd with the desktop before I head out.

Unison works great for this.

How much do I need to sync? Seems like there's files scattered in various places.

I don't want to sync EVERYTHING in /home/<user>/.kde4, since I maintain different setups & data for other apps, etc. So i need to pick and choose; There's

/home/<user>/.kde4/share/config/{kmailrc, mailtrasports, kontactrc, etc}

and,

/home/<user>/.kde4/share/{kmail, kontact, korganizer, etc)

also, there's

.kde4/share/apps/kmail/{dimap, imap, mail}

do I need to sync all of those too? /dimap contains local cache, iiuc, so isn't that regenerated by kmail locally? would I need to back /dimap up, too?

This advice is derived from moving data between machines rather than syncing; I would envisage problems trying to sync IMAP and POP accounts because of the different ways in which they work but POP mail is in
/home/~user/.kde4/share/apps/kmail
The calendar data is in
/home/~user/.kde4/share/apps/korganizer
and I don't think you will have any problem syncing this data.
You can find where your address books are by right clicking on the Address Book in Contacts and looking at the Address Book Properties. You may also need to look at System Settings>Personal Information>KDE Resources to check the type of Address Book because I have ended up with different Address Book types and different locations for the storage of the contact data of different users.
How easy it will be to sync address books, particularly if they are different types, I don't know.