After watching my Tiel Mali flying amongst the tops of 50metre high gum trees, I'm convinced that you cannot ground a Cockatiel, particularly since Mali had one half opened tail feather at the time, no other flight feathers or tail feathers(she was just a bit ugly at the time!!!), but jeepers could that baby fly!! Not trying to get away at all, infact when she flew into a lower tree my partner climbed up it and she ran halfway to him then thru herself at him, very cute. And very reassuring that my little girl still loved us!
I would suggest arranging things so that your birds can be flighted, perhaps you can put their cage in a place that doesn't have a direct route to outside doors? But at the same time I understand how paranoid you can get about your birds getting out. After losing my Tiel Birdie out the door, I still can't decide whether or not to try and clip Mali, I'm so so paranoid about her getting out the door. Our family is very very conscious of where the birds are at all times, but Mali has a nasty habit of trying to follow you out the door. She'll wait until you're nearly out and then fly to you, and she's so fast too!
I'm too scared to start working with her with her Feather-Teather(which she may or may not be too fat to fit into currently

), because then I'll want to take her outside, and if she got off she'd be GONE!! That bird is gonna give me a heart attack i swear