Depends on the recipe and how long you plan on keeping it around. I wouldn't be too worried about getting enough bitterness in there. With the magnum and centennial at 10 min, you're getting there. The longer you age the beer, the more the hops, bitterness, flavor and aroma, are going to drop out. Better to start out with too much and end up with a nice beer in two years than good now but too malty a year from now. A beer this big should age for at least a year before it really starts to mellow out.
What's the malt bill?