I use a 90/10 mix of distilled and white vinegar to rinse and subsequently flush my loop,
but bottom line is yes, clear all the crap out of the rads that you can, blocks may have tiny residues but nowhere near the amount of crud that sits in the rads
use boiled, not boiling, boiled water to fill the rad 4/5 and thumb over the ports, shake like a mofo for five minutes and drain, fill 1-2/5 and repeat, do those steps twice then fill 100% full, shake and drain, rinse with distilled and leave to dry,
don't block the ports with tissue etc as that leaves the moisture in there instead of letting it evaporate into the room, it can rot your rad worse than if you hadn't flushed it
Moto