yes, correct. The Dithered mixer does not just add dither to the master output of a mix. It dithers the whole system as needed.
If your aux send on bus 1&2 is post fader, then you are sending a 48bit signal into a 24bit bus (all bussing and inserts in HD are 24bit), and so you need dither.
Sometimes you will only notice a subtle difference, other times you will notice a big difference... but regardless, you should try using using the dithered mixer unless there is some reason you would need to do otherwise.