I have a 21 Eurail pass and have made reservations on all train connections requiring them except one -- Luxembourg to Paris on the TGV. For flexibility reasons, I'd prefer not to make that reservation until the day before or even the day of. Anyone think that's too risky?
For travel alone or with one other person, you are probably fine. The day or two before will usually work, and if you don't get your plans too packed and you have to travel one day earlier or later, no worries. If it is in August I would try to make them at least 3 days in advance just because everyone in Europe goes on holiday in August.
I'd be a bit nervous but then I haven't been on the TGV for a couple of years. It is a really popular train service but it depends which is more important to you the flexibility or the reservation.