Yes, it is often discussed.
Yes, you should be concerned about personal safety. But this concern should be reasonable, and it should not prevent you from going. All you really need is common sense and situational awareness.
Take a partner if it will enhance the enjoyment of your trip. But a partner will do very little if anything to increase your safety.
Run-ins with yahoos does happen (more often to women of course), but such experiences are very rare and can almost always be handled with a calm attitude, a bit of humor if necessary, and firm assertiveness. If you do have such an experience, it will likely only be annoying and not dangerous. Staying out of sketchy situations as much as possible will reduce your risk greatly. On the Northern Tier route, there are no sketchy situations.
Your friends and family, along with my friends and family, and in fact everybody's friends and family, never think this is safe and never will think this is safe. It's just the way things are. Thank them for their concern, and then go anyway. Their fears are a million times overblown. They'll get over it. Honestly, it's no more dangerous than moving around your home town.
P.S. If this is your first long tour, and if you want the comfort of a lot more touring cyclists on the road with you, consider the TransAm rather than the Northern Tier. I've done both.