High speed train reservations

Usually 3-4 mos out is the best time to buy to get the best discounts - I think with the TGV you can book 4 mos out. I'm not sure that I've paid more buying mine online other than perhaps paying an extra euro or two for using my credit card. Of course, keep in mind that you are buying tickets for a particular time and train.

If you buy a normal ticket for a TGV, a reserved seat is included. Reservation is only ever an extra process if you have a pass.
And the earlier you buy your ticket, the cheaper it is.
Where you are when you buy a ticket is irrelevant, just ensure your credit card company does not refuse a transaction booked in France (and your credit card may charge extra for a foreign transaction, wherever you physically are).

If purchasing directly from SNCF you should get the lowest price for the day of travel. Buy as early as you can to save money and worry. Major routes often let you print out your ticket, containing your seat reservation, at home. Others can be picked up at any SNCF station or boutique. The hidden saving is time; line-ups for purchase from station agents can be lengthy. You can sign up on the TGV website for notification of promotions, some going on sale much earlier than regular tickets. Buying ahead costs you flexibility but saves a lot of stress as well as money.

We bought our tickets from Paris to Bayeux online several months ahead, as soon as tickets for our dates went on sale. Definitely cheaper and I was able to reserve specific seats. I went through the SNCF site and printed out tickets at home. No one checked our tickets on the trip to Bayeux, but conductors came by and checked everyone's tickets during our return to Paris. We had no problems at all. I think it's unlikely for trains to be sold out, but you'll pay more the day before travel.

Pass holder reservations are not particularly cheaper in Europe and are prone to selling out in France. In the past year I did see a fair amount of availability just a few days ahead for TGVs, but a few weeks ahead is certainly a safer bet, especially on routes with less frequent service. It's also easier to book them at https://ricksteves.raileurope.com/us/rail/point_to_point/triprequest.htm than to wait in line in a train station.

Thanks for all the good advice! I've decided NOT to get a Eurail Pass and I have the info I need to make the necessary reservations. Love this forum!