It depends on the person and the dog and their relationship. My first dog died at home in his sleep after a series of strokes. We were planning to take him to the vet that day b/c we kind of thought it was close to time; he just took that burden away from us. *misses her Jackpot dog* I was really down for several weeks afterward, but he was also my *only* companionship much of the time. I still occasionally tear up if I see a dog that looks like him, and I have one of his toys in a box in my closet. I'd say mourn as long as you need to, and public opinion be darned. She's your dog, your friend, your responsibility: feel how you feel.
I got somewhat used to being present for euthanasia when I was with the humane society in college, but it never really got to be easy. Usually those were severe neglect or abuse cases that necessitated an end to the suffering, so it was a blessing in a way, but I did (and still do sometimes) mourn for those little lives. I'm a crier, but then again, a lot of them never had someone care enough *to* cry for them, so I'm not ashamed of walking out of the exam room bawling with an empty cat carrier. I also don't see death as a bad thing, or something to fear. If the animal is suffering, why extend that? It's not fun and I certainly don't get any joy out of it, but in the case of very old or sick or suffering pets, I think it's often the right thing to do.
What's usually suggested is that you pick her 3 favorite things to do (play ball, sit on the couch, meet you at the door, whatever), and when she can't do them any more, make the appointment. Until then, love on her a lot and spoil her rotten, just for the sake of your own heart. She already knows she's loved, but it seems to be easier on the humans if we've done all we can to show her that near the end.