I pop all babies, usually twice (once when they hatch and then just before I ship). Popping is not 100% accurate on females, but it is close if you are experienced. It is generally 100% accurate on males, unless they somehow got mixed up in another cage after sexing. But that is very rare and should get caught when they are sexed before shipping.
Once a corn gets to around 16" or so, I switch to probing instead of popping. I am not comfortable using that much pressure to evert the hemipenes, as would be required on a larger juvie.
If there are any shows in your area, stick your little guy in your purse and ask an experienced breeder to show you how to probe him. If they are not too busy, they should be willing to help. I do it all the time at the shows, as long as people are willing to wait until I am not busy with customers.
Everyone who wants to breed should become proficient at both popping and probing. Once you have some practice, don't ever take anyone else's word for the sex. Always check yourself immediately after purchase. Anyone COULD make a mistake - if there is a error, you want to find out right away before it is too late to fix it.