I will be taking the train from Cherbourg to Paris, probably on a Thursday or Friday, mid-day, in mid May.
Do I need to make a reservation and/or pre-purchase a ticket, or will I be able to get a ticket at the station in Cherbourg the day of departure?
You will have no problem with getting a ticket for that day. The only advantage to early purchase of the ticket is the possibility of a discount. That can be off set by being locked into a certain schedule. Trains have lots of capacity and are rarely sold out.