We all know that seams need to be pressed. Although there still seems to be a lot of discussion about which direction they should be pressed. From a longarming point of view, the seams should lay flat, regardless of the direction in which they were pressed.
Ironing the back will also allow you to determine if there are excessive threads that could show through on the top.
Ironing the front makes a nice smooth top. But ever more important, it can highlight where the seams are open or not properly secured. After all, you don't want a longarm's foot to get caught in a seam and do some real damage to your quilt top.
After all, we want those works of art to be the best possible - stellar!