My Nanday was only trimmed when I first got her. But, since she couldn't fly, she would jump off her play gym when she wanted to come to me. I was worried she would get hurt. She's been fully flighted for 11 years now. When she does start flying, I can just put my hand up and she will land on it, even at the vet's. She does usually behave there, but every once in awhile will say, ha, ha, I can fly around in circles up near the ceiling and you can't get me! We let her for a minute or so, then I will put my hand up and she lands, and that's the end of that.
My BC was always fully flighted, but she never did fly much.
My amazon was trimmed by his original owner when I got him. But, he's molting now and new wing feathers are coming in. I will just have to wait and see what he's like once he can fly.
If you have any concern though that the bird could get outside and fly off, keep it trimmed would be my suggestion.