First set a suitable default for your p elements globally at the start of the document.

e.g.
p{margin:0 0 .7em}

Then when you want extra margin on say the last paragraph or a special paragraph then just add a class.

p.last{margin-bottom:2em}
<p class="last">Lorem ipsum dolor etc....</p>

There is never a reason to add an empty element just to make space. You would very rarely use breaks in paragraph sections either as breaks are used in things like addresses, poems, form controls but not just to make space or end a line rather than using a paragraph.