As the other poster said it is usually the person with the best perspective and in most cases, the person that caught the disc is usually going to argue that they had best perspective.
Which is complete buttkus... the player who caught the disc is concentrating on his hand and the disc... his eyes most definitely are not on his feet and the line. I've always let the defender call my feet (or called it as the defender). What we leave to the offensive player is when he controlled it (i.e. before your foot left the ground or before the back foot came down).