I still haven't reserved any hostels for my stay in France.
I'm not lazy, I just like to play it by ear.
I'm thinking of one night in Hon-Fluer,
some other couple nights in Lyon and Annecy...
I think for most places thats fine, however if you plan on staying in Paris, book ahead, for the giant city it is, there are not a lot of hostels to begin with, and a lot of the hostels in Paris are not very nice.
Also I'd just check out a site like Hostel World, enter in the place you want to go, and just check out how many hostels the town/city seems to have. If there seems to be a lot, no worries, however if there are only one or two listed, you might want to book ahead.
If you are staying in Paris, we stayed at St. Christopher's Inn which is a hostel we booked on hostel world and we really enjoyed it. Safe travels!
cant reccommend in paris.