I'm going to drag this next one out of my distant work memories from a job long past. It does, however, remain valid today, even as 'snail' mail becomes less common in the face of electronic communications.
As the diagram above shows, there is a certain expectation that one might have of where everything on the outside of the envelope will be found. The stamp, you will notice, is up in the top right corner and it is well-aligned with the edges of the envelope.
My job experience came in the form of a horrifying experience I had, having delegated the sending of a number of letters to a colleague. I watch in utter disbelief as this person applied the stamps to the envelopes any which way: crooked, even upside down! I didn't say anything at the time — we had just competed for the same position and I, as the one who got it, was feeling a little odd about delegating things to this particular colleague. I quietly resolved just to send my own letters in order to avoid a repetition.
What's the big deal? I feel like the attention one pays to the elements on the outside of the envelope are a cue to the recipient as to the degree of care I am taking with our relationship. Slap-dash means I really don't care about you, and I would never send such a message to anyone. Being an uptight WASP, I wouldn't send that message to my worst enemy.
So if I should ever have the occasion to send you a letter, you will see that the stamp is as straight as my failing eyesight and trembling hands can make it. I only expect the same in return.