When you find it fits your style.

A general rule of thumb is that if you see the ground, normal photos tend to look better if they are straight rather than angled.

But if you are doing fashion, macro, or some other style then it can be benefician to angle the camera.

Photographing highrise buildings against a sky without the ground visible can absolutely be improved by adding an angle, especially if you can capture the sunlight in a cool way by doing it.

A fashion shoot can absolutely be improved by having SOME angled photos, it can give life to the scene, say that you are taking a photo of a model in a multistory building, you can absolutely get some great photos by shooting down or up through the stairwell at an angle so you can see the model walking up/down the stairs like you only catch a glimpse of them.

Macro photography has the advantage that you don't notice the bigger world and you just focus on the small subject, as long as you get the focus right (preferably with a shallow DOF) the angle matters very little.

If you are concerned about keeping the camera straight, see if you can activate a digital horizon on your camera, I can do that on my old Lumix GX80, but I have not found it on my new Lumix S5.

Assuming you mean left and right tilt and not up/down. That would make it "Dutch Angle".

https://www.diyphotography.net/why-tilt-the-camera-the-history-and-use-of-the-dutch-angle/

It's generally used artistically to show tension or unease.

it really is up to how you want to frame the photo and convey the scene. it's an artistic choice, really.

basically, does it look good when tilted? yes? tilt it! line that horizon up with the corners! make a dutch angle, create tension! fix the leaning tower of pisa! the world is your oyster; tilt that too!

hey, in this world of digital, you can always take multiple pictures at different levels of tilt, and even rotate and crop the image in an image editor afterwards.

don't stay stuck in not adding tilt; try things out, experiment, find what you like. if it turns out you prefer the order and beauty in a simple and level photo? more power to you!

I'd say it's used semi frequently in skate photography. Typically a close shot with a wide lens. The tilt might be to realign the viewer with the skaters body position and show the extreme angle of the trick.

When taking photos of the leaning Tower of Pisa ¯\_(ツ)_/¯

I'd say that when the viewer can tell it's tilted on purpose. Otherwise it'll just look unprofessional.

The only example I can think of is this one photo I saw which was on a 45 degree angle so that the shadow in the middle of it was straight. It was obvious why it was tilted.

When taking pictures of a lounging cat so you can snap a closer photo without cropping out any of the cuteness.

in 1994

For a portrait, when she is sufficiently drunk.