How exactly bows are explained as being limbered or unlimbered as needed has always been handwaved away in my experience (at least since I have been led to believe it isn't possible to keep a bow strung for very long periods as I understand it). It would take a round or three to string a bow when going from total non-combat to combat, with worse for crossbows, but most groups just handle them as always ready.
as an odd-side note. My Hollow Earth Swordworks crossbow is of their earlier design. Nobody mentioned unstringing it when I bought it. It still shoots with about the same strength, and it's been strung for like 10 years.
On the same string.
Granted, it sees less use. But just sitting there? Getting cleaned a couple times a year? It can be fine.
some qualifications:
my x-bow, undrawn is under minimal tension, it's not bending as much. The way the did the ends, makes it harder to unstring than the new design.
a long bow, strung, is under higher tension as the bow is bent far greater in its idle position. So I wouldn't leave a bow strung like that.
I have no idea if people marched around with a bow slung all day (given that it's easier to carry over your shoulder that way, and to use it in a hurry).