Symbolism, Imagery, Allegory
If being fallen or "slack" is a sign of one's state of sin, doing the opposite – standing – is a sign of goodness, innocence, and distinction. So, for example, in the passage from Book 3 above Milton's God says that he created mankind "sufficient to have stood," which sounds a lot like "with sufficient capabilities to stand tall and do the right thing." We see the same type of language in Book 7, where Raphael talks about how one distinguishing feature of man is that he is "not prone" (506); in other words, he walks upright, unlike the animals. When Satan first sees Adam and Eve, Milton tells us that he saw "Two of far nobler shape erect and tall,/ Godlike erect, with native honor clad" (4.288-289). In this passage, the fact that Adam and Eve stand "erect" (or upright) is a sign of their distinction as (innocent) human beings; it is what separates them from the animals, and thus makes them closer to God. For another passage, check out 11.509.
In addition, the action of standing up can mean something like what it does nowadays in phrases like "why don't you stand up for something?" So, in Book 9 Adam encourages Eve to keep her reason "erect" (9.353), by which he means something like "stay sharp and do the right thing." In Book 5, when Abdiel realizes that Satan's rebellion is a bad idea, Raphael says he "Stood up, and in a flame of zeal severe/ The current of his [Satan's] fury thus opposed" (5.807-808). At the end of Book 6, Raphael says of the rebel angels: "firm they might have stood,/ Yet fell" (6.911-912). And of course, in Book 3 God explicitly contrasts standing and falling: "Freely they stood who stood, and fell who fell" (3.102). And let's not forget that when he was in Heaven Satan was "upright and pure" (4.837).