Do not apply the tabindex attribute to non-interactive elements (headings and paragraphs, for example). Doing so will add an additional, unnecessary tabstop for keyboard users.
A common mis-perception is that "screenreaders need to be able to access headings, so we must add a tabindex". But screenreaders can access headings perfectly well without tabindexes.
An unneccessary tabstop can confuse users, because any attempt to interact with the element (using SPACE or ENTER key) will provide no feedback to user.
An unneccessary tabstop can increase physical discomfort for some users - for example if using a binary switch rather than a traditional keyboard.