bongo: as robby said, "experiment."
it's really a matter of personal choice.
i started dryboxing cigars only when i noted that they were taking a LONG time (like over a month) in the humidor (at 70%) before they were smokeable. i would only dry box certain cigars that i now know from experience tend to have a very tight draw (perdomo especially).
what i do now is keep my humidor at 65-68%. for most cigars, they'll smoke just fine out of the 'dor. if i do meet up with one that STILL has a tight pre-light draw, i'll dry box it overnight. if it's still tight, i'll keep it dry-boxed for a day or two more. that does the trick 99% of the time.