The traditional rule is seventy-seventy: seventy percent relative humidity at seventy degrees Fahrenheit. It is a serviceable rule that has been in print since well before any of us started smoking. It is also, by the lights of most modern enthusiasts I respect, a touch high on both axes.
The case for lower humidity is straightforward. At seventy percent RH, a cigar’s wrapper holds enough moisture that the burn slows and sometimes wanders. At sixty-five percent, the burn is brisker and more even, the ash holds longer, and many smokers report flavours coming through more cleanly. The Cuban official guideline for aging finished cigars is sixty-five to seventy percent RH at sixteen to eighteen degrees Celsius, which is firmly on the cooler, drier side of the older rule.
My own humidor sits at sixty-five percent at sixty-five degrees Fahrenheit. It produces a more consistent draw than anything I have held at seventy-seventy. Boveda’s own positioning is that sixty-nine percent is the middle ground broadly preferred for premium cigars; sixty-five is for those who want the drier draw and longer ash, and is a fine choice for long-term aging. Seventy-two has its uses in drafty humidors or genuinely dry rooms, but I would not aim for it inside a tight-sealing cabinet.
The worst thing for a cigar is not slightly too wet or slightly too dry. It is bouncing between the two. A humidor that sits at sixty-eight percent year round will produce better cigars than one that swings from sixty-two in winter to seventy-four in summer.
The full case on humidity, temperature, beetle thresholds, and cold-climate stability is in How to Rest a Cigar.