I'd certainly prefer stainless bleed nipples over brass, due to the superior strength of stainless, as Eric says. You will still get galvanic corrosion between the aluminium caliper and the brass bleed nipple, but the brass will shear much more easily. I'd suggest that the best approach, as Peter suggests, is to exercise the joint by bleeding the system at regular, and reasonably frequent, intervals to nip any corrosion buildup in the bud. I'd also apply a wipe of anti-seize compound (aluminium or copper) to the bleed nipple thread during assembly.
