It seems to better to follow the 'theme' of the race, rather purely better stats. For example, a knight should probably a Breton, because they're the France or England of the setting and have technology from around the 1300 or 1400s (late medieval)
The Imperials are cultured but morally dubious (like all Catholics and Mediteraneans), with Renaissance level technology. They're also stooges for power, unlike the free-willed Bretons (not to mention, they have nice gothic black hair) For these reasons they suit being Assassins, no matter what the stats say.
Nords belong back in the dark ages, they're true barbarians (and I'm sick of chilly Skyrim)