Exceptions: There are certain types of formatting that typically work well if directly applied, and don’t necessitate the creation of a new custom paragraph style. Good examples include adding italics, bolding, and underlining.
It's important you provide your readers visual cues to separate one body paragraph from the next, otherwise paragraphs blend together and create a horrible reading experience.
For the body of your book (everything after the title and copyright page), either use first line indents at the beginning of a paragraph, or use the block paragraph method. Don’t mix both first line indents and block style for the main narrative of your body.
Most authors will use centered block paragraphs for their front matter (I discuss front matter in Step 21 much later), and then first line indents for most of their body.