I have the Herm Sprenger Bow Balance stirrups and they do have some play in them where they can go on an angle. Though it's not like others out there that can noticeably do it.
But I have found that my heals are down as soon as I put my feet in the stirrups and the don't come up.. unless I do it.
I LOVE THEM! Haha. If you're getting some, go with the Herm Sprenger ones, most definitely.They have like links in them that help you keep your heel down majorly. The cheaper ones are just broken in the middle and the rubber can be cheap and crack and break. But if you're on a budget, go ahead, but keep in mind quality lasts.
They ARE miracle stirrups for me! I have tendon problems and without them can barely get my legs into a correct position at a halt with someone pulling on them, and had to sit and roll my ankles for nearly 5 minutes before I got off unless I wanted to collapse when I hit the floor, it hurt so much, but things go much better with them. They've really helped my ankles and knees and let me relax through my legs because I'm not in pain. I've also found they've eliminated the awful problem I had with losing my stirrups at every opportunity.
But I'd definitely go for sprenger or something equally reputable. I've tried budget versions and they made next to no difference.
Safety stirrups themselves come in different kinds - are you looking at the ones with the rubber band or the 'bent leg' ones? I've heard bad things about the former, such as the band pinging off at inoppurtune times and the iron bending out of shape when under the stress of jumping. I think they're for kiddies really, where the weight or strain on them isn't going to be as much.