Its optimal to use your sholder to parry the right hands from the oponend. When you parry and he is overextending with his rights you are gonna take his head off... But this movement has his price. You must have gut timing and reactions and if you miss the parry its bad news for you(watch Archie Moore vs Yvon Durelle). Blocking, as like with the hand, is much more solid, but it limits your options for counter because you have shorten the movement from the oder guy and you can loose balance. You can block easy left jabs with your sholder too. Normally that does not compromize your defence much, but dont over do it because you open your self to right hands to the body(watch Toney)... The position of the hands leeds to the cross arm defence wich is mostly cross parry... Very good example to watch for both mistakes and good use simultaniously(lol) is Ken Norton. He is pretty solid when you try to hit him with straights. He take them all with the sholder or the right hand but this leeds to cross parring. In cross parry you are open for the left hook. Than to block it you must lift your right elbow up
or return the glove fast to your right temple. Ken dont do both of them good(Norton vs Forman, Norton vs Shavers)... Good example of the cross arm defence is Evander Hololyfield and of cource Archie Moore. Personally i dont lilke to go in the cross arm (i am not verry good there). I like to go in a slide crauch to the right(which gives a slight engle to the oponent and you are not on his line of attack and in general the position outside of the left hand of your oponent is "safe zone") and bobing from there(watch Rocky Marciano). You can go under all punches and you are loading the right hand. How you are going to stay depends of your build... I am very solid(110 kilos most in my legs) and not very tall(6'~5'9"), so crauching deep make alot sence for me. If you are tall watch Ezzard Charles, Bob Foster and Mike MCcallum to name a few. I hope that this helps.