Prague to Munich: recommended to pre-buy RailEurope ticket?

Going Prague-Munich April 27. Only 2 trains are scheduled. Can I expect problems if I don't pre-buy ticket through RailEurope? Can I risk buying at the station? Will price be different? This is listed "open seating", so I'm guessing I can't pre-reserve seat, so I think that would mean there's no limit on tickets. So I'm guessing that means whoever boards first gets best seat. So a RailEurope advance ticket lessens the wait in line. But I have to pay extra to rush the ticket to me before I depart April 24. Can I just risk it and show up at the station? And how much in advance do you have to get there? Or can I purchase there a couple days in advance leisurely? Are all prices going to be equal to whatever RailEurope quotes? They seem to go up closer to trip. Is a week away about the max. they'll go anyway? I like to be flexible and not buy non-refundable but don'twant to be stupid and stranded.