Being cautious is always a good idea, especially when, as dogster suggested, the "McDonalds Coffee" syndrome is always with us. I do think, however there might be a better solution than making a blanket statement about extension cord usage.
Surely the good folks at Cookshack have some idea of how much voltage loss each product can handle before there's a serious degradation of performance. To that end, I've created a voltage drop table for the 08 based on wire lengths of 25 to 100 feet in 25 foot increments and wire gages from 12 to 18. Here's what it looks like.
If the folks at CS could publish the magic number for each product, then just enter the table for that particular product, go to the intersection of Wire Gage versus Length and find the voltage drop under those conditions. If the drop is over the Cookshack recommended, don't use that size and or length.
I would be most happy to provide similar tables for the 09, 50, 55 and any other model for which the target drops were specified by the factory. The calculations for this table took about 5 minutes. The graphics took a little longer.
PS. Actually, the 08 and 09 would be on the same table and the 50 and 55 would be on another.