It very much depends on the operator; Eurostar and SNCF requires very expensive seat reservations for all TGV services, but hardly any services in the German-speaking countries do so. More important than the price though is the availability - only a small proportion of seats are open to Interrailers on the French trains, so if they sell out you could find yourself unable to board a train that is still half-empty!

The situation in eastern Europe is more variable, but the reservations aren't generally as prohibitive as in France.