For the record, I do hear and feel a nuance of meaning change between such verbal phrases as 'straighten a closet' and 'straighten up a closet.' If I straighten up anything, I'm aiming for the high side of my straightening capabilities, which are sorely limited, let me assure you. If I'm straightening a room, I'm making it look in reasonable order on my scale, which is by far lower than average. But if I'm set on straightening up a room, the level is higher because the word 'up' suggests the heights. Up is [oh, let me holding my ridiculously-beating romantic's heart] a word of spiritual ascent with wings upon it. To straighten something is one thing, but straighten up something is different, yes, in the very nuance you all were discussing. Something straightened up is glowing with order. And if I've straightened up something, then on the scale, I've attained the heights for me and probably just about average on normal folks' scale.
We have here in the States the cola that makes you feel up seven days a week:
7-Up
