I guess I wouldn't keep heavily rusted rings--there are probably some that wouldn't use ANY with rust, but I don't mind a little. I guess I'd be afraid of flaking some of the rust in the food I was preserving.
I don't see any issue as long as they screw on/off without too much difficulty. The main problems are just a) aesthetic, and b) if they get too rusty they can be hard to get properly snug-but-not-overtightened, or can loosen the canning lid when you remove the ring the day after processing.
Personally I don't think there's any other health issues involved - the ring does not contact the food itself anywhere, and even if a wee flake of rust fell off into your jelly as you were opening the jar, I don't thnk it's gonna kill you
Removed from the jars the day after canning, however, and washed/dried/stored properly, they will not rust for many many years, IME.
I use steel wool, then after dry will oil them with vegetable oil and let set. The oil keeps the rust from returning. I can then wash immediately before using anytime I can. Just what I've always done.
If you lived near me, I'd give you a bunch! Being a fairly new canner and a bit compulsive about it, I have a gazillion rings. I think they are breeding in my drawers. All shiny and new.
Next time I need lids I'll buy lids with rings and take better care of the rings.
Any craft ideas for rusty old canning rings?
If I can't find another use for them they will go to the scrap yard for recycling.