I have an unpadded Maya ring sling and it's my main go-to sling for out and about right now. With that, and with ALL my other unpadded slings (ring and pouches), I've never had a problem with the rails digging into her.
The trick is to make sure that the rails are all the way up in their knees. They need to be froggied, with knees higher than bum. The inner rail can be tightened quite a bit to get it up there. And like a PP said, it's bunched more towards the knees than towards their upper back. I usually start with most of the sling and pull it up over their back, rather than starting behind them and pulling it down under them. This way, their weight in the sling is mostly sitting at their bum and their knees are just kind of resting up there.
If it's TOO tight it will dig in, DH has had the issue a bit with his pouch sling, which (now that she's almost 3) is starting to get a bit snug with her. And if their legs are not froggied enough, so that the rail is on their thighs instead of inside their knees... well, then their weight is not sitting properly into the "pocket" of the sling, and all their weight will be resting on the rail, and it will need to be extra-tight to hold them there anyway... so yeah, THEN you'll get digging in.
Probably the most common error I see, when I happen to see other babywearing mamas out and about, (other than having the sling up at their neck instead of over their shoulder) is having the kids sitting with their legs dangling pretty much straight down and the sling barely covering their bum. The mums are always having to hold the kid by them bum to keep them in place... kind of defeats the purpose of 'hands-free' eh?
Anyway, all that being said, 90% of the time the rails don't seem to bug the kids even when they do leave red marks. It's not enough to actually be painful or cut off circulation.