Saliva cortisol tests are abnormally low. Serum cortisol tests are normal. Dr says I don't need an acth stim test due to serum results. Agree?

Yes. If your saliva cortisol tests are low, then you have adrenal insufficiency. Acth stim test will only help determine if the cause is in the adrenal glands, pituitary gland or hypothalamus. Without other hormonal abnormalities or abnormal levels of sodium or potassium, this test is generally not needed and treatment for the adrenal insufficiency can be done.
More info. Salivary test was low when? Am, midnight? Serum test was normal when? And what was the values? If you are trying to see if you have adrenal insufficiency, it is very important to find out the cause and make sure the diagnosis was done correctly. Discuss this with your doc, see a specialist if necessary. Do not take steroids until a firm diagnosis is made or if life threatening.