Not trying to say you and scrod are wrong, but, when talking about nipples, aren't we talking about that gram at the rim? And more particularly 16 grams at the rim (assuming 32h rim and roughly a half gram each difference between brass and alloy). Still, if you figure a rim/tube/tire combo (oh and don't forget velox) is probably on the order of 6-700 grams on the light side, so the weight difference is only 2-3% at most, so I get where you are coming from in that it's not going to make a significant difference (especially if you're not a fatty like me and want to use lower spoke counts). But even if you use a 48h tandem wheel, 24 grams is a tiny percentage of total bike+rider weight rotating or not.