The bad thing about a nose down position is sliding forward, so I like my saddle nose up just a tiny bit.
I did notice that the further back one sits on the saddle, the less likely a nose-up angle would interfere with the soft spots. Of course people are all built a bit differently so ya gotta try different positions to find one that works best.
This is true about nose down to a certain extent however I think that this comes down to the severity of the angle more than anything else. For example I have tried nose down -3 and -2degree approximately just to kind of see what it was like and in those cases I was definitely sliding or falling forward. I could feel myself tipping so to speak. However, when I ran the saddle nose flat or up to about ~1degree max I couldn't really notice any slipping even when ridding hands free and purposely trying to test this.
I think some people go a bit overboard with the nose down as a method to relieve pressure and in those cases yes slipping will be an issue.
Also, where you sit I feel like effects saddles that are not perfectly flat in particular. Those that are flat as a board, Antares comes to mind, should be the same angle the whole way through while something with a dip like the Aliante it will change markedly with nose angle