Being tied is something all horses should learn.
But I rathereither use a collar around his neck (made for horses) or a thin string between the halter and rope; if he panicks and rears, the preassure from the halter can damage a nerve-center in the poll if the halter or rope desn't break. Also never tie in a rope halter by the same reason.
If your horse has to learn that it shouldn't pull to get free (if it has noticed that the safety string breaks ;), use the collar. It's not over the nerve center.
First times tying up, its better to have a long rope through a ring, you holding the other end untill the horse noticed that it's stuck, but that it's not dangerous. Give some rope if it gets too stressed.
But.. letting it stay tied for a while is good, if I hck out and brig a picnick, I want to be able to tie my horse to the tree while I eat :P