Many of the places I am reserving for August 2009 are asking me when I will arrive. I don't plan on buying my train tickets until arriving so I am not sure what to tell the hotels/guest-houses. How should I go about planning this? Thanks for your help!
It's not that important so, just give them a time. I would suggest to fill in a time, at the beginning of the evening.
If you are planning on arriving late send them a mail that you will arrive late. (in your case, send them a mail, if you know you have late arrivals.) Most hostels will keep a bed/room free for you until a certain time (depending on the hostel/place). Some hostels mention that on their website, others don't. And they do want to be informed if you arrive later than that time.