I think that it could be done, but it would not be preferable. The cockatiel would probably have trouble climbing, especially if they were vertical bars instead of horizontal. There would also be the chance that the bird could accidentally get its head stuck. Once I kept budiges in a cage with 3/4 inch bar spacing and one budgie became an expert at squeezing through the bars and escaping, so it could happen! All in all, 1/2 inch bars are usually a better choice because it allows the bird to grip and climb easier.