I think it is enough, to book them a couple of days in advance, on the internet, before u go to a place. It would save u from having to try at more than one hostel, if u arrive without booking, because they do not always have beds available.
If it is high tourist season(july), then maybe book them as far ahead as possible.
Thanks everybody!!!! Ypur comments have been really helpful! I havent decided when I will be travelling yet, probably July/ Aug! I will do a bit more research and probably book the first on online and then play it my ear! I dont want to restrict myself, but I also want somewhere to sleep LOL. I have considered night trains and plan to encorporate both trains and hostels.
I am getting so excited