Here are some deep words from one of the master technicians of cyclo-cross (Adam Myerson -who else?):
“You ONLY step through if:
1. You’ve clipped out of the left pedal already and are riding on the arch of your foot.
2. It’s a fast approach to the barrier so you have sufficient time to set up and coast.
The default dismount is to clip out of the left first, and just step off around the back with the right foot. That works every time, in every situation.
Option 2 is to stay clipped on the left, and step around the back with the right. Turning your hips as you step off is the key to releasing the left pedal. This is good if you have to pedal all the way up the barrier, or are dismounting on a hill or in mud where you pedal until the last moment.
Option 3 is stepping through, and it’s something you should use somewhat rarely. Unfortunately, because it’s the “fastest” dismount, people treat it as the default, and end up doing it still clipped in on the left, and losing teeth on barriers.
NEVER step through if you’re still clipped in on the left NEVER NEVER NEVER!”