P-end-tag is only required in XHTML, not in HTML.

Correct

But some times you have to close it any way eg. when you align the paragraph left/right/center.

Incorrect. The only time you need an explicit end tag is when you want to end the paragraph and immediately follow it by something that is allowed inside a paragraph (such as text or an inline element). This is usually a bad idea.

Would it for any reason be a bad idea to mix the usage of P-end-tag

Only that consistency is a virtue which aids in code maintenance.