I've been using knee sleeves for quite some time now (5mm Rehbands, not super tight). I think they are 100% safe for the knees...
I wear them for squats only and slide them down to my ankles for the rest of the workout. I do have them on during warm-ups already (after all, you're trying to warm up :P) and keep them on when I rest between sets. I don't wear them on my Deadlift day.