For the record, this question is asking whether or not you need to invert 90 degrees to actually get the full benefits of this therapy. And, the answer to this is no, as the benefits can be achieved at virtually any angle. Of course, though, at a mild angle such as 20 degrees, you will not fully decompress your spine but you will still feel the benefits overall. Once you start inverting up to 60 degrees, you will then fully decompress your spine and reap the benefits that way.
All of this being said, though, a lot of users will opt for 90-degree inversions because it provides the best feeling. There is absolutely nothing wrong with that and again, it really comes back down to your preferences.