I agree that you need to taste the finished product before you make a flavoring salt addition decision.
With that said, I may move through kegs more quickly or be less sensitive to oxygen exposure to the head space than some of those with more refined pallets than mine, but once it's on tap, if you think it could benefit from some gypsum, by all means, boil a half cup of water with the gypsum to sanitize, let it cool (covered), and add it to the beer. I have done it before when I wanted to make a flavor adjustment and was very pleased with the results. As long as you're not trying to save the beer for a special occasion in the distant future, you are unlikely to notice the effects of some oxygen exposure in the process.
Just my opinion.