I have blue/red rings and use them to color code all my IEMs (because I'm lazy and can't be bothered to spend time determining which side goes into which ear).

I've never specifically used them to change the tuning, but I have done it to help with insertion depth (the ring keeps the tip from being pushed further on the nozzle, thus the tip seats further into the ear).

With a lot of my IEMs I put the rings such that they don't effect the tip's seating, but I may have to try this with my IE300's and see if I notice a change in sound. I just put a ring on and yes, it does work to change the seating position of the tip on the IE300's. The tip is held further out towards the end of the noozle and yet is still held in place by the retainer lip, so seems pretty darn secure (which might be an issue with some IEMs).

I haven't yet listened enough to compare the sound difference, but it's interesting that it's an option.