You can try to dryhop with cascade after primary fermentation if you want a spicy earthy flavor, but with a bitter wheat beer, I would use amarillo if you want to dry hop. It's a little more flowery and it will complement your centennial bitterness. Now, as far as flavor is concerned, I would definitely let it go another 3 weeks before making any decisions on what it will turn out to be like. Normally you would let wheat beers condition for short periods of time before serving because they tend to get off flavors (atleast in some peoples opinion) when you let them go longer, but in this case where it is more bitter, the hops will preserve the original flavor and you should condition it a little longer, like say 2-3 months. I think you will be just fine if you let it ride for 2-3 months and add a little amarillo, not much, just like .5 oz in like the last 1-2 weeks.
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