When should you season sauce that needs to be reduced?
First of all, people indeed often add some seasoning in the beginning, but are usually careful with the amount, especially with salt.
Flavor-wise, you could add it at the end, after the reduction and be fine.

However, salt can also affect the process of cooking. For instance, it can draw moisture out of some vegetables when frying them first, meaning that they will be cooked a bit faster. That is why some salt can be added at the beginning. At the end, after the reduction, you should taste and add some more seasoning, if needed.