first of all, a coil "tap" is not when you split a humbucker into single coils. some rare old single coil pickups were made with three wires, a hot from one end of the windings and a ground from the other end [like the wires on a normal single coil], and a third wire that "tapped" the pickup windings at some point halfway through. using this extra wire "tap" off the middle of the pickup would give you less output and a different sound.
i've never seen a humbucker with the windings tapped halfway through. to avoid confusion, most people call splitting a humbucker "coil cutting."
since you don't say what 5-way switch you are talking about [JEM/RG570?], i can't answer your question directly, but i can explain a little about coil cutting. there are two ways to wire a coil cut, depending on which coil of the humbucker you want to be on.
using the DiMarzio wire colors you mention, the white soldered to the black is called the "series link" [because it links the two humbucker coils in series]. if you connect the DiMarzio series link to ground, you activate the "slug" coil of the humbucker, the coil without screw head pole pieces. on a bridge humbucker, this is the coil farthest away from the bridge, the inner coil.
if you connect the DiMarzio series link to hot, the guitar output, you activate the other coil, the "screw" pole coil. on a bridge humbucker, this is the coil closest to the bridge, the outer coil. these two coils do sound very different from each other, mostly due to their distance from the bridge.
on standard JEM and RG's with HSH pickups, the 5-way switch is wired to automatically activate the "slug" pole coils, the inner coils, of the two humbuckers in the 2 and 4 positions. this is possible with a stock 5-way because the switch has an extra set of poles that isn't used in normal Strat-type wiring. in the full humbucking positions 1 and 5, the coil cut is not active.
for some neat coil cut schemes that i've designed, check out http://www.his.com/sha3u/gear/wiring.html
. for more info on wiring humbuckers, see http://www.stewmac.com/wiring101/