In general, it's nice to have the 'selectCUOnLoad' set to true. But on some sites, it might make sense to explicitly set to false.
Examples:
YouTube main page
Here, when I load the page, I'd like the focus to go to the search box (instead of the first video on the page getting selected).
(Pulkit brought this up a few months ago.)
YouTube specific video page
It'd be nice to be able to go through the comments on pressing j,k on this page. But the first comment shouldn't be selected on page load. (That would scroll the page down beyond the video which would not be desirable).
In general, it's nice to have the 'selectCUOnLoad' set to true. But on some sites, it might make sense to explicitly set to false.
Examples: