I haven't built my d16y7 for nitrous. The motor is pretty close to stock aside from the basic bolt-on crap. Just keep your motor tuned and stuff. I think the biggest killer of motors, with or without nitrous, is not maintaining the motor. Check your stuff after heavy usage. And if you do happen to break parts, I've got a y7 with a 100k on it coming out in a week or so.

edit - It's not purely safe because it's not idiot-proof. For instance, you could always spray at idle or something dumb but I know you won't do that... I would hope NO ONE would do that. LOL. At $25-35 a bottle refill (10 lb bottle), you'll be sparing with the n2o. Nitrous will not get you to and from the grocery store quicker.