The crease is noticeable in bright ambient lighting conditions like outdoors in very bright sunlight or if glare from lights hits it at certain angles. For example, imagine you have a lamp and you take the shade off of the lamp. If you stand so that the lamp is behind you while you are browsing the phone, it will glare off of any normal phone screen. On folding phones, the glaring will make the crease visible.

In addition to bright lighting conditions, it's most noticeable when not viewing the phone from head on, like you said. Viewing the phone head on seems to minimize the opportunity for light to bounce at an angle and so the crease is often not noticeable.

If you use the phone normally (viewing head on, as in browsing content, writing a text, etc) and in normal indoor lighting conditions, the crease is often totally invisible to the user.

If you work outdoors all day in bright sunlight, tbh you are going to notice it. But then I find most phone screens are hard to use in bright sunlight anyway because they aren't bright enough.

Tl;Dr - There are many cases where you notice it, but there are many more cases where you don't. It just sort of blends into the background after you use it for a few days anyway.