Why are you doing acid rest if you have acid on hand?
I don't know how would it effect pH predictions at sacc temps (if you're using one of calculators), and can they count it like you added it later..
Correct, it doesn't change much. 122 is hot enough that everything that is going to dissolve has dissolved (for the most part). You probably already know or would guess, but want to add your salts and stir out any dough balls before you measure.