When should I defoliate and/or prune?

What organomann said. Though, there are some reasons to do it despite the negatives he lists that are all true. Risk reduction being one. You should have a specific reason to do it. Leaves overlapping that cause standing water on leaves is a disease vector. A shit ton of congested leaves raising local RH is a disease vector, too. Standing water and excess moisture on leaves, bad! Unnngghhh!! https://youtu.be/Mawve_3NSj4?t=30 When i see a leaf controting or can easily predict it, that's another reason, also interelated to congestion above. Controrting means the leaf is putting a lot of effort into seeking light - reacting because is not getting enough. Rather have a few fully covering area without contortion. Vectors of light not hitting tops of leaves are severely attenuated, even if still causing some photosynthesis. Greater surface area of leaves lying properly is probably better. Timing helps avoid any negatives too. Be pataient and let any growth that is expected to fill in gaps is actually be big enough to do so. No reason to take it off too soon. I also remove axillary buds that I know will never be included in my canopy, but I don't clip off that node's leaves, if there is no reason to do so.