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.