How to "composter" a print-at-home train ticket?

Speaking from our one-time experience on the Paris-to-Bayeux train, it was not necessary for us to do anything with our home-printed tickets before boarding the train. On the return trip, conductors came around and checked everyone's tickets. I presented our printed tickets, which they checked with a handheld device, and had no difficulties.

It is not necessary to validate TGV tickets, since those are sold with reservations. Other tickets are validated at the station, at a machine on the station or on the platform.

Sometimes an agent in France will sell a pre-validated ticket, but I think that would only happen at the station and if you told the agent you were going to take the next train.

When ever I bought tickets on-line that required validation, I did not get print-at-home tickets, but instructions to retrieve tickets from any SCNF ticket machine with the credit card used to make the purchase. Kind a blew up in my face when my chipless credit card would not work in the machine. Fortunately, at the ticket window they found a register that could read magnetic strip cards (not all of the registers work because they never get used with magnetic cards) and I got my tickets.

A bar-coded ticket valid only for a specific train does not need to be punched or validated before boarding. If it's non-exchangeable or non-refundable, that's another clue to the same rule.

cgichard, The purpose of "composter" (stamping the date and time on the ticket) is to stop you using a "open" ticket twice on different days.
It is a waste of time for a ticket that is already only valid on a specific train, because you can't use it twice!
And, what is the point if stamping a print-at-home ticket when you can print out as many copies as you want? Print-at-home tickets have the train, seat etc. on them, that is what stops you using them twice.

To avoid the issue Sam had, when buying tickets that you cannot print at home, be sure to select the "pick up at a French station" option. In addition to stations, you can pick up these tickets at any SNCF Boutique. These are like "train ticket stores," and are all over France (Paris has many locations). You take a number like at a bakery, and sit in semi air-conditioning, so it's much nicer than standing in lines at a station. When your number is posted, you go to the desk and show them the print-out you got when you booked your tickets; they swipe your card and print your tickets, and you're set.

Thanks to everyone for the helpful replies. Having made more bookings now, I see that it is only for Eurostar and TGV journeys that print-at-home tickets are offered on my booking site (loco2.com) and I shall stop thinking about composting those. For regional trains I have a code for the self-service kiosk that I can use to collect my pre-paid tickets from any SNCF station. I have done that before without problems, and greatly appreciated the opportunity to print out all my tickets that way ahead of time when I wasn't rushed or making a tricky connection.