My husband has a fib. He was told that to only seek medical help for heart rates over 90. Is this always the case?  

Rate control. It sounds like he has atrial fibrillation. This is when the atria (the top chambers) of the heart are beating uncontrollably. This becomes a problem when these beats get transmitted down to the ventricles. Medication generally is aimed at slowing down the heart to prevent this. This is what the doc is talking about with him, most likely.
Often so if.... Many people live healthy lives for decades in af which is a stable rhythm that is often permanent. If he has been evaluated for causes and associated conditions at least once, determined not to need anticoagulation, or if needed, is receiving anticoagulation, and if he feels well, no further medical treatment is indicated.
No. It's complicated. Chronic a-fib is considered "rate-controlled" (not requiring immediate attention) up to about 115 beats per minute; certainly, beyond that, medical attn must be sought. Many people, though, don't tolerate rates that high due to other conditions like coronary disease or heart failure. He needs attention any time he experiences symptoms (chest pain, palpitations, dyspnea). Every case is different.