galvanized or stainless turnbuckles for exterior wire rope fence?
The age-old question of whether or not to use alloy materials is subjective.


cost vs. longevity
consequence/cost of failure


Rest assured that the galvanized components will fail. You will then be unable to adjust the tension of the cables. If the consequences of the failure outweigh the initial investment for alloy materials, you should buy stainless (this includes the cable itself as well).

For decades and decades the oil industry resisted the use of alloys in piping and vessel construction due to cost; it was just easier and cheaper to replace carbon steel components at their earlier end-of-life. Now, the safety and environmental costs of failure are beginning to outweigh the cost of investment for alloy materials, hence they are now "alloying up".