Cut and dried is a useful expression for conveying that something is fixed and (for most purposes) unarguable. If you get it wrong and write cut and dry, an argument will undoubtedly happen.
Dried herbs picture from Shutterstock
Here's the definition of cut and dried from the Macquarie Dictionary: "fixed and settled in advance". Also of interest is the speculation about its origins: "from the practice of cutting and drying herbs, which then lack freshness."
We're not adherents to the theory that what a phrase meant once determines its meaning forever, because that's simply not true. Language changes. We're mentioning etymology in this case because it might help people who use the wrong version remember the correct version. The herbs have been cut and dried. They weren't dry to start with.
Because language changes, it is possible that at some point in the future so many people will get this wrong that cut and dry will become the accepted expression. It is possible, but it hasn't happened yet. Until then, you need to get it right when you write. Accuracy matters.
Lifehacker's Mind Your Language column offers bossy advice on improving your writing.