My advice would be to look at their past sales and pick the auctioneer who does best with the type of material you want to sell.
Generally the consignor fees & terms are similar. Best to focus on the "right house" than the "best fee house" and end up with a mismatch between your material and their bidder base. The 8% you save on commission will be completely offset by a 10% lower realization.
I've consigned to almost every major (and "not-so-major") auction house in the US and have never had a problem with any of them. It's a highly competitive field, so if you have truly exceptional material you're in a good position to negotiate. For not-so-exceptional material, fees & terms are pretty consistent, the biggest exception being those willing to run lower value lots typically charge higher commission rates (logically so.)
Finally, many houses have minimum consignment thresholds. If you're just trying to move some odds and ends (you said "sending in a few of my stamps") and they don't amount to at least a couple thousand dollars (actual realization,) it's possible that few will be interested, or if so, will charge you a higher commission (so much so that places like
ebay or BidStart will be better, and much quicker, options.)