Yes it is. Obviously, this is just my anecdotal experience, but the first time I tried to install Linux the troubleshooting documentation I was working off of only mentioned eth0. I tried to configure everything to eth0, and nothing worked. I gave up and swore off Linux for years. Eventually, I tried Ubuntu, where everything worked, and figured it was a driver issue that Ubuntu had solved.
I got around to trying out Gentoo, which has amazing documentation that really drills down into what you're doing and why. That's when I ran ifconfig and iwconfig for the first time and realized I probably would have been using Linux years earlier if I had known how to find out that my NIC was wlan0.
So I'd say referencing eth0 throughout is fine, as long as at the beginning you take the time to point out that it may not be eth0, and more importantly, how to find out what it is.
On the surface it would seem like you're correct, but only so in some cases.
If for example you're running Linux on a Macbook Air you have no eth0 by default. Ergo, documentation referencing it is technically incorrect - it would be possible to say badly written at this point but that's even more pedantic than your comment ;)
