I am interested to see what everyone thinks about this. I know this is contrary to what many people do. If you don’t agree I am interested to know why, but it has to be more than, “That’s how I learned” 
When learning to shoot a rifle if you are you going to use it with a red dot, than irons are a backup system (certain 3 gun applications excluded). I think that it is important that you know how to use iron sights correctly and are proficient with them. But if you are brand new and you have both a red dot and irons why would you train 50% of the time with irons and 50% with the red dot when you will probably only use the irons 5% of the time if that. It makes sense to train the most with what you use the most.
When learning to shoot a rifle the basic fundamentals can broken down into aiming the gun, holding the gun, and firing the gun. Aiming the weapons consists of sight alignment and sight picture. Of course with sight alignment the basic concept is to center the front sight in the rear sight and maintain focus on the front sight. To someone that has never fired a weapon focusing on the front sight while trying to hit a target that is out of focus is a very foreign idea. How am I going to hit something that isn’t even in focus? An additional conceptual difficulty is teaching someone how precise that front needs to be centered. Misalignment measured in thousandths of an inch can be the difference between a hit or miss or a poor group.
An advantage of a red dot is that there is less to do with it. There is no sight alignment. Sight picture is a much easier concept with a dot. Where you put the dot is where the bullet goes, if everything else is done correctly. If you have never shot a rifle before it would make sense to learn on the easiest sighting system first and then move to the more complex. But many people have been shooting before red dots became popular and first learned on irons. I believe that most teaching is still done this way. Learn to shoot with irons first and then add an optic. It is much easier for a new shooter to remember put the red dot where they want the bullet to go than it is to remember perfectly center the front sight in the rear sight, bullet will impact at the very top edge of the front sight, maintain clear focus of the front sight but still place that sight centered on a blurry target.
Although irons are important, a person could learn to shoot a rifle only using a red dot sight and be perfectly proficient because they may never need the irons. This isn’t a perfect scenario but it is conceivable.
For the trainer or instructor their are advantages. By initially training with a red dot it eliminates one variable in diagnosing a problem. A poor group could be attributed to poor sight alignment or poor sight picture, among other things. With the red dot the possibility of sight alignment is removed. A red dot is also much more forgiving of head and body alignment. The red dots are essentially parallax free and will not be impacted by moderately changing stock to cheek weld.
An analogy to this is teaching someone to drive. Like shooting, when learning to drive there are many things that must be mastered simultaneously. You have to control the accelerator and brake, steering, watch other traffic in front and behind you, etc. If you were teaching someone to drive it would make sense to first teach that person on an automatic transmission. Once they are proficient with the basics move on to a manual transmission. The same concept applies to a rifle. Start with the easiest to use setup which requires the least multitasking than move on to the more complex.
The only advantage of first training with iron sights is monetary. For someone that is buying equipment iron sights are much more economical than a quality red dot.