NS Trapper have you ever seen the fur you send in that way come back from a tannery?? I doubt it!! Once the leather drys and sets that fur in you can't tell if it's going to slip or not, however I'll garauntee you that a black or dark green belly will be bald at the end of the tanning process.
Gritguy, your right; I haven't seen fur come back from the tannery. I skin, flesh, make any necessary repairs to, dry and ship to NAFA for roughly 70 trappers. I should say that the only pelts I have seen "black belly" on is coyotes; particularily snared ones that were caught in warm weather. All long haired furs I do get borax around the ears, armpits and heads as well as a dusting on the belly if green. Pelts showing green belly get the bleach/water treatment as well.
Before skinning the critter, I give the pull test on the belly area. If only 2,3 or so hairs come out the critter gets processed. If I have hair coming out in steady patches and clumps then the critter goes on the carcass pile. While I don't buy fur, like fur buyers I have seen it all - roadkill, deer rifle shot coyotes, etc. When stuff comes in frozen I have no idea how long it hung around before it was frozen. You never know what you will be skinning. I have seen coyotes and cats with bellys the colour of a green pepper with no slip or smell at all; and have skinned others (mostly yotes) that looked fine yet smelled like they sat around for a week in 90 degree heat.
What I have seen however are the grade sheets for my customers, and other than the odd pelt that was beyond repair or natural damage such as bites in rats or old scars etc on beaver (these will show up as "VSL" on your sheet); the fur grades just as well as the other fur.