Tetanus I believe is most usually associated with a piercing injury, particularly by rusty old metal. Like most clostridials, it can be introduced by inadequate antiseptic practises when injecting or ear tagging, or through any open wound such as are occasionally found with elastration of tail or testicles.
Historically it has been thought that tetanus is most likely to be found in soils where equines graze.
As Rosemary says, whether or not to vaccinate in flocks where none of the clostridial diseases has been experienced is a choice. If or when the first death occurs from a preventable disease, how would you feel? If you'd beat yourself up and wished you'd vaccinated, well then, perhaps it'd be better to vaccinate anyway! If you'd be able to accept this loss (or losses) and probably review whether to vaccinate going forwards, then that's a valid choice.
Vaccines are permitted under organic regulations, presumably because the preventative use of vaccines is preferable to avoidable deaths or needing to use antibiotics in the event of a preventable disease. In non-organic farming, current guidance is to regard vaccination as the default choice in order to reduce the need for antibiotics as well as because in commercial flocks the economic case for vaccination usually makes itself.
However, most of the guidance regarding vaccination does relate to commercial flocks, where density is usually higher, and stresses more frequent. In low density, low stress management systems such as many smallholders will practise, the choice to not vaccinate in the absence of a history of preventable disease is surely a valid one.