Most cockatiels are unfortunately... They only bond to one person/bird and once that bond is made everyone else kinda sucks in their heads. It took Spike about 6 months to allow Joey to pick her up when she crashes or not bite him when he tries to give her a kiss. But thats with him being around her every day
If you get cockatiels young they can be great with all people. The young cockatiel Dante who I found up in San Francisco is great with new people. After I tamed him(took about 3 days) I brought him with me to stores, petco, my friends house, and car rides. I let him meet new things and people everyday and he now lets anyone hold him and pet him. Older birds will be harder because they're already set in their ways, especially smaller species. Trick training and a healthy diet can help eliminate a nippy birds biting if done correctly.
and a random bird rant inserted here lol
I also don't baby my birds, many people think they're fragile and it makes the bird expect to be treated that way. This isn't to say I'm rough with them, I just don't act like they're going to break if I flip them upside down play games with them like you would with a dog. Another thing is I never force them to step up, many people press against their chest making them off balance and stepping up to catch themselves. People will also use the command step up, and if they don't they force them to step up to "re-train" them. Even small birds aren't stupid, they know if you want them to step up if they've been taught. If they don't want to they shouldn't be forced to, once a bird feels like they have a choice they mindset completely changes. Nippy birds will bite less because they don't have to resort to biting to tell you they don't want to do something.
Here's some photos of Dante. I gave him to a friend of mine who has a fully flighted female cockatiel and he is having a blast and also talks.
Dante doing recall training
Also I taught Rosie to wave