Welcome to CS jase.
Both work just fine and it doesnt matter which you might use (rubber disc or metal blind filter).
Yes follow the instructions and MAKE SURE you flush effectively with clean water (and re season the group by making and discarding a couple of coffees) at the end so there is no detergent residue to upset your delicate taste buds when sampling your next coffees *;)
Dont forget to do "the jiggle" before doing the backflushing.
If your machine is not relatively new and it has never been back flushed, it would be better to physically remove the shower and clean in there by hand before backflushing with detergent......otherwise you may be at it for a considerable time. Cleaning in behind the shower first will in that case make back flushing an easier quicker job.
Of course, when doing anything "physical" with your coffee machine dont forget to first pull the plug right out of the electrical socket. Did I say FIRST ;)
I was just down your way on Friday.....flew out just in time.....
very first CS site sponsor