I personally am not a fan. You should be aware of the environment your wireless gear is sitting in.
If your in the middle of nowhere and only have one or two access points, then you could leave 80MHz If your in a crowded metro area, I would use 20MHz If you have lots of access points in an office design, I would use 20MHz
Yes and no. If your APs support 80MHz channels, they may end up using it. That could be a good thing or a bad thing depending on your environment. I personally never use auto, not for Cisco, Aruba, or anyone. I look at the RF in the space and decide if there is room on 5GHz for 40MHz and if not, 20MHz it is. I find there to be very few scenarios where 80MHz makes sense. The thing to understand is that 80MHz uses 4 20MHz channels. That will limit the availability of non-overlapping channels and make CCI a potential issue.
That depends on the environment. In high-density deployments it's recommended to stay away from wide channels due to it being harder to avoid co-channel and adjacent channel interference if you have less channels (which is a direct consequence of having wider channels). In most corporate environments 20MHz channels are used.
Definitely read the points about default channel width in the docs below: