The position on your Defense Team is what's important since when enemies raid your defense runs automatically. Attacks from raiders generate mana for the defending heroes and in general the center position is hit most often.
Because of the number of hits the middle takes, you generally want that hero to be tanky. So if your middle is a healer, then they can heal the damage they can heal a lot of the damage they take. If your middle is a damage dealer, they could be throwing their attack regularly.
A lot of defense teams tend to be based on the nature of the center. For example, a center healer may be flanked by two big damage dealers so any missed shots up the likelihood of dealing big damage. Conversely, if the center is damage, the flanks might be buffs, healing, or riposte (if you are running a fast kill style of defense).
So in short, center is important and your friends are correct.