The danger of booking in advance for the best fares is that those fares are date and train specific and non-refundable, non-exchangeable as of the day of travel. If you book a train too close to the scheduled arrival of your flight and it's late, you could lose that ticket and have to pay full fare.
I've been an hour late to Germany on at least 25% of my flights. Once I booked USAir to Munich and then started to check arrival times. In the first week or two it was 8 and 10 hours late into Munich! Yikes.! Fortunately, by the time I flew they had their act together, and I was on time. But it can, and does, happen.
I always try to spend the first night close to wherever I land, somewhere accessible with a "regional" fare (Länder-Ticket). For instance you could go to Bacharach the first night (a worthy destination, on its own, and much easier with jet lag), and then take the discount fare train to Paris the next day. The discount fares can be booked from Bacharach to Paris as well as from FRA to Paris.
If you book in advance from someone like RailEurope, you will actually pay more than full fare for your ticket. Except for those dangerous train specific advance purchase fares, which you can only get online from the Bahn, you'll get the best fare possible at the airport when you land. For a trip that long, I would want to get reserved seats. I've only once seen an express train that was SRO (on a holiday), but there were plenty of reservable seats.
