i usually end up waiting around 2 or so weeks before even looking at grafting with seeds, gives the seeds a little more time to get used to being alive, and get that little bit sturdier... I def agree with the climate theory, and the hardening them off after the humidity plays a big role as well... The only lophs i've lost are due to coming straight out of the humidity tent and into the general atmosphere - they shrivelled pretty quickly...
fatter stocks are pretty easy, i've had around 95% - the ones i've lost i either knocked off after the graft took, or i left them in the humidity chamber to long and they went to mush...
None of my ario/peres have taken as yet - i've only tried 4 of these though