Depends on the page design and target user. For lengthy, multi-section pages, a "Back to Top" can be helpful, but it'd be better if there was a fixed Table of Contents with links to the sections of the page. Otherwise, I always have the Home/End buttons.

On mobile, page should be designed such that the button is easy to reach, visible and not blocked by or blocking the text. For a button placed on the bottom right of the page, the body content should be set up so there's a little extra padding on the right. This way, there's a clear visual separation between the text and the button.