At risk of being pedantic, and also noting I'm not a materials engineer and look forward to being corrected, a reminder for this discussion that "strength" means a lot of different things, and the "strength" of FDM prints is significantly anisotropic---much more robust against some forces in some directions than others.
For example, say I FDM a whole body tube in the natural way (circle on the XY, long axis of the tube along Z). It's going to be pretty robust in normal ascent, because it's compressive on the Z print axis. The thrust and air resistance are pushing together what's essentially a stack of plastic rings, so they're working against the compressability of solid material. In contrast, karate chop that tube on its side, the XY print plane, and multiple things could happen: 1) It could much more readily shear between layers, the bond between layers along the Z axis being generally much weaker than a uniform extrusion on the XY plane; 2) It could crush if the circle's arch and the XY structure (perimeters and infill) can't handle the force.
Maybe YOUR rockets aren't subject to spontaneous ninja attacks. But I'll bet they do suffer from, say, sometimes descending at an angle, such that a rear tip hits the ground and is subjected to force in the (more or less) opposite direction from the rest of the rocket still continuing down. Setting aside other problems like crushing, penetration, etc., I'd expect such a rocket to crack between layers in this kind of scenario much more easily than any problem the ascent phase is going to impose.
The main point is that a fin canister is almost certainly going to be printed with the body circle and fin protrusions on the print XY. So it'll be much stronger against thrust than it is against other forces, especially landing and transport. I can readily envision a 0 infill thick fin that I'd expect to fly fine but be very brittle to landing & handling. There could be other problems as well, e.g., does any shock cord mount design integral to the canister lead to compressive forces or tensile forces? I'd expect the latter to be more challenging for this kind of printed part.
Conversely, a large fin printed on its own standing up, i.e., with the cross-section on the XY and the root parallel with the print plate, would be subject to forces in flight (shearing) in precisely the part's weakest direction. Avoiding this is the same reason you cut solid wood fins with the leading edge aligned to the grain, so flight forces are against the grain rather than with it.
So it's useful to be explicit about what kind and direction of strength we're talking about, and how the part was printed, in terms of orientation as well as parameters.
