Which means the average damage is either 5.625 or 8.25. If you round, it's either 6 or 8 points.
There's no point in rounding average (mean) damage, since it's a already an expected value over many rounds and thus is meaningless for any given round. Median, yes.
Since there's no reason to use a rapier over two short swords except to use cunning action to withdraw from the enemy, which is sort of a skirmisher fighting style, there's even less reason to dual wield two daggers over two short swords, or use a single dagger over two.
Basically, rogues dual wield in 5th edition if they want to land their sneak attacks reliably. It's their only option since they don't get extra attacks.
The real question is, is it even effective to use cunning action to play a game of cat and mouse, over the bonus action damage for dual wielding, which increases your average damage more and more, the more sneak attack dice you get.
I wish they gave rogues a 19-20 crit range with daggers at level 5, and made sneak attack incompatible with dual wielding. Either you attack once, possibly with advantage for two chances to hit and deal sneak attack, or you flank and get one chance. Advantage is better than sneak attack, but not much better except for crit chance. There is a rapidly diminishing benefit for more d20 rolls after two, to trigger SA damage.
When you have advantage while dual wielding, the chance is vanishingly smaller of foregoing sneak attack compared with single wielding for 4d20 vs 2d20.