It's been my experience that the biggest thing that causes cracking or unraveling is the differential between the humidity inside and outside the humidor. If you live in a fairly humid climate it shouldn't make much of a difference whether you store them at 65 or 70%. But in a very dry and hot climate the wrapper can split in a matter of a few minutes. Here in Denver the strong sun seems to aggravate the situation.

I remember reading a paper by the guy who invented the cigar oasis. He determined that once a cigar gets over 65% humidity the wrapper becomes much less porous and at about 67-68% no moisture can pass through the wrapper. I reasoned from his article that a dryer cigar (65% or less) can adapt quicker to a dry environment since it seems to make sense that a more porous wrapper would allow the cigar to breathe and equalize quicker. I equate it to not making slits in the top of a microwaveable pouch. If the steam can't escape the pouch will expand and eventually pop. My own experience seems to bear this out. When I went from 70% to 63-65% (smoking RH) I experienced much fewer problems.

Temperature swings are extremely important as a couple degrees difference can change the RH inside your humidor and your devices usually can't keep up. This is probably why temperature controlled humidors are so popular among serious collectors. A 3 degree F drop can increase the RH about 7% if moisture is not pulled out of the air. Beads are probably the best way to control RH in a non temperature controlled environment, but I have my doubts as to whether they can actually keep up.

I have also read that cuban cigars smoke and taste much better at about 65% where non cubans seem to taste better at a higher humidity. Dry boxing can help, but the inside of the box should be about 50% and the cigar stored at 70% should be given at least 3 days to equalize. I have found that this seems to be true.

As for long term aging conventional wisdom says to store at 70/70 but bring them down to your preferred smoking humidity (usually by dry boxing) before smoking them.

I am no expert, but the above seems to make sense and has worked well for me.