Cast iron in the dishwasher?!?! OH THE HORROR OF IT!!Yeah, okay.
Actually, it won't hurt it, but it will take most of the "seasoning" out of it, and you'll have to reseason it. i often don't even use soap when I clean my cast iron skillets, just hot hot water and lots of "elboy grease". Then dry well, and wipe down with a very light coat of either mineral or vegetable oil.
I've heard those old wives tales too, and to tell you the truth, I bought into them too for a long time. The simple fact is that they ain't true. A lot of folks have a particular cast iron skillet that is used exclusively for cornbread and they never give that skillet more than a wipe with a paper towel. I'm not going to convince them and we'll just have to disagree. So be it.
Seasoning in a cast iron pot is lard, fat, oil, that seeps into the pores of the iron. It imparts an almost non-stick surface to the metal. A well-seasoned cast iron pot is a thing of joy. Nothing sticks to it, but from time to time (every five years or so) you need to thoroughly clean the metal and re-season. My pots, and most of the cast iron I've seen, gets a crust on the outside. This crust is oxidized carbon from food splattering on the outside of the pot. Eventually, you'll look at it and gag, then it is time to clean the pot thoroughly. Washing it in a sink ain't gonna do it. You have to burn that stuff off, in a hot fire.
So, you go outside and build a fire. When I lived in the country, I'd use a brush-pile of various hardwoods and deadfall I had accumulated over several months. I'd put the pots in that fire and light it. When everything had burned and the ashes had cooled, I'd pull the pots from the fire. Occasionally, one would crack from the heat, and that is just part of the charm.
But, all that crust would be gone. Then I'd take them inside, give them a good washing with soap and water and re-season them. It's simple. Give the inside a good coating of Crisco or other solid shortening, then put them in a 450 oven for a half-hour. Take them out of the oven, let them cool and repeat the procedure. When they've cooled a second time, take them out, wipe them with paper towels and store them away.
I'm convinced that what makes a cast-iron pot non-stick is the smoothness of the interior surface. The seasoning (shortening, oil, fat, whatever) floats out during heating and keeps food from sticking to the surface. If the surface is smooth, nothing can stick to it. If the surface is rough, then food has an opportunity to stick.