Could be tricky for Andy. Hopefully one of those two has an early loss in one or both of these Masters.
If Andy is seeded 9th for the US Open, his last 16 comes against the guys seeded 5-8, likely to be Berdych, Ferrer, Raonic or Dimitrov. If he sneaks into the top 8 he will face someone seeded 9-12 in the last 16, which would be something like Dimitrov, Nishikori, Gulbis or Gasquet (I'm assuming Gasquet will rank ahead of Isner by then as Isner is defending 600 at Cincinnati). Either way, Murray will be faced with a quarter final with Djokovic, Nadal, Federer or Wawrinka and can't face those guys at any earlier stage.
The first of the two mentioned quartets of potential L16 opponents is certainly trickier but you could also end up with a draw where the 9th seed is set to face Ferrer in that round and the 8th seed is set to face Gulbis or Nishikori. In that case you'd possibly prefer to be the 9th seed. Who knows? I think it's more of a big deal to get in the 9-12 bracket than the 13-16 bracket so you don't have to face the very top guys in round four. It will still be interesting between these guys, though.
In Murray's case, I don't think it makes too much of a difference whether he makes the top 8 or not. If he is in any kind of form to contend for the title, he should be winning against any of those potential opponents, apart from possibly Berdych (who has always caused him trouble) or Dimitrov (who we don't really know will back up his grass court performances). On the contrary, if he plays like he has for most of this year, then any of those opponents have a shot at him.
Top 8 seeding would be a nice bonus and come in handy though for sure, whoever does get it.