If you wanna book all your places now, then I say go for it! As for making a time of arrival, I would say 4pm is a good time, and if your going to be late, get to a phone and call the hostel to let them know, should not be a problem.
I used theTravelpunk Hostle Reservation System
to book my rooms on my last trip to Europe and It was great, everything was ready for me when I arrived. And its a great way to support the site!
And a very big
to you!!!! Its great to have ya on the boards!!!