If you are worried about the auction house having a bad day, then you've taken it to the wrong auction house. You take it to the auction house that you have confidence will get the best price for your items. If you have premium items, the ignore the commission rate. There is a legitimate reason a great auction house charges a relatively high commission.
All the major auction houses take a commission off both seller/buyer, and it's no longer the 10% cut of yester-decade. So they're going to make their 25%+ on the hammer price. The cash offer will not be unreasonable in most cases, but if you want a hint at how much of an ADDITIONAL hit you are taking by accepting their cash offer, just consider their seller's commission (which they will lose if they sell their own inventory).
If you are set on selling through a major auction house, then there are really only 2 things to consider regarding cash offers from auction houses (my opinion):
#1. How bad do you need the cash? If you MUST have the money within 1-2 months, then take the cash offer.
#2. Is it a "hot" area? The China market is still considerably better than average, even if it is not Cultural Revolution era items. You are very likely to meet or beat the estimated auction values. You won't lose out by risking an auction. The only issue is #1.
Again, my non-expert opinion.
Best wishes!

k