Plants aren't as fragile as you might think when they are budding.
I've heard a lot of bro science on the topic, such as a tiny amount of light leaking into a grow tent will cause a female to herm, which isn't true. Think of night time and all the stars in the sky. Say a full moon is on display, too. I've never once seen or heard of a plant herming because of too many stars in the sky, or a full moon bringing enough light to cast shadows on the plant.
Anyways, like I said, plants aren't as fragile as people think.
You can most definitely defoliate during flowering cycle. I always walk away from my plants with leaves that were covering other flower sites. I try to give every potential flower site as much light as possible.
A lot of people set defoliation times during flower. I've heard quite a few people say they do a heavy defoliation at the end of week 3 flower and the end of week 6 flower.
I hope this helps.