Hostels in South America

[wheelecdx]

I'm going around South America from December through until February, and I'm wondering whether it is necessary to book hostels in the major cities e.g. Buenos Aires in advance, or if we can turn up on the day and get a bed somewhere. Could someone please shed some light on this query, as I don't want to end up booking these hostels online, and then being left stuck to a strict schedule whilst backpacking around? In the same way, I don't want to be left homeless if bookings do need to be made in advance!