Red-shirting is very, very common in our small competitive community. When we moved in to our house, the first conversation that we had with our next-door neighbor (whose twin sons have nearly the same birthday as our DS) was if we were planning to red-shirt our son or not. He was only one year old at the time!!
I was told when I was pregnant with Pud, due in August, that if it were a boy, I just HAD to hold him back on NOT on any condition put him in K at 5. I remember thinking, gosh, let's let the child be born before we start categorizing him/her.
However, I will say that with some boys with summer birthdays, I can see why parents red-shirt them. Some of these boys don't have the maturity or emotional readiness to sit through full-day K at barely 5. I'm sure that also applies to girls, but I've known more boys that way than girls. A good option for us was a part-time, church K. (Granted, Pud was doing 2nd grade work at the time but he loved it and the teachers loved him, so it was a good fit, socially). Not quite half the kids went on to first and the rest repeated K in public school for many reasons.
I think it all depends on the child.
