Unfortunately, the back bearing in the engine is basically in the front part of the engine.

If you're going to fly the model sooner than later, it really doesn't matter which way. Back when I had lots of wall space, my models were alternated (and a couple were hung from the ceiling). Was there any difference? any problems? yes....

For models that didn't get flown for long periods of time, the nose down ones would gum the engines. Not badly, but it's worth mentioning. And the ones hung from the ceiling got more dust on 'em. Again, not a biggie.