Sarastro - I think I made an error like you say - re-checking I see no IC trains that require reservations. Good!

At sncf.com, the Intercité trains between Paris and Brive, for example, do require reservations. There is a note next to the schedule for each train that says "réservation de place obligatoire". If you look up the same trains on Capitaine Train, this is not stated but seat assignments are made at no additional charge.

Maine - those are the trains I was familiar with from Austerlitz that I thought required reservations - so some do most don't.

Night intercités train do require reservations.

I'm in Carcassonne at the moment. I took the TER train from Toulouse yesterday. We left at 3:55, the ride was only 1 hour, good scenery, no reservation needed. Call it the "milk run"! The train was not crowded at all. So the TGV option isn't the only one available.

But thankfully I did book Prem tickets from Montpellier to Marseille before I left home via Capitaine.com. The cost was only 10 euro each vs the current price of 60 euro.

The normal fare for Montpellier to Marseille is only 27-44 euro tomorrow (ie, no notice), I don't see any that cost 60 euro normally, at least that's the fare second class. The best one, the direct TER, is only 29-32 euro. So 10 euro is a good deal, but the usual route isn't that expensive.

I'll admit I never rush to buy these 3 months in advance, either, and have never had a problem getting one later on at a good rate. I just don't usually have my plans that set so far in advance. I think that kind of advice on Fodors just can worry some people that they have to rush to the comptuer at the crack of midnight exactly 3 months in advance or whatever, or they are out of luck, which isn't true. In fact, I'm pretty sure in a couple cases the fare actually went down a bit, but that's not common.

I'm also pretty that that some years ago, they never did have PREMs except on the longer TGV routes, adding shorter routes like to Rouen, etc., was something that came later. And the whole purpose of PREMs is to sell seats, so they often don't have them on real popular routes where they would never need to.

Sorry, that was a typo. It was 20 euro for the 2 Prem tickets, Montpellier to Marseille.
It was the only tickets I purchased ahead of time so we could be in Marseille to catch our flight home. Otherwise we are taking TER trains since our plans are never set in stone.