SapperPE said:
I not only use it on my professional stuff, but also on my birthday cards, Christmas cards, and informal emails to my family and friends... I even introduce myself to random strangers as SapperPE, P.E., just so they know my credentials. Okay, no I don't do all that.
I think both are acceptable, like M.D. or MD. I personally prefer P.E., since they each stand for a different word.
This topic reminded me of a story my father used to tell about a co-worker of his. The co-worker had his PhD in geophysics and was one of those pompous a-hats that insisted
everyone should address him as "doctor". On a business trip he apparently made a point of this with the staff of a hotel they were checking into. To his surprise he was woken up in the middle of the night by the staff asking for his assistance when a woman staying at the hotel went into labor.
Moral of the story, it's good to convey your credentials, but be sure it is appropriate for the situation.