As far as I understand, the phrase "per se" comes from Latin. Yet, it has become to accepted in English and is used so commonly, that I was wondering if the phrase is still considered of foreign origin and requires italicization or it is already considered everyday English and should not be in italics.
The phrase "per se" does not have to be italicized.
A general rule of thumb is that if the foreign word is in an English dictionary, you do not need to italicize it.
Use italics for foreign words that have not been integrated into the English language. Words like alumni do not need to be italicized, but words like semper fi do need to be italicized. The difference is that alumni is understood by English speaking people as graduates of a particular university, whereas semper fi requires translation to be understood.
This rule, like several others, is open to interpretation. A general rule of thumb is that if the foreign word is in an English dictionary, you do not need to italicize it.