I just thought about how seasoning a sauce is not a wise decision because if it needs to through reduction like marinara then the flavors would get too concentrated. But I always see people put salt and pepper into sauces before they are properly reduced so maybe there is a reason to season before a big reduction?
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.
If by "season" you mean salt, then add it after the reduction. Depending on the recipe, there are herbs and spices that you want to cook with the liquid. Reduction is the evaporation of water and does result in concentration of flavor...because of this it is very easy to get a result that is overly salty if you salt and then reduce.