When to bi-wire?

I heard that if there is a large impedance difference between upper and lower drivers then bi-wiring is better. Can this just be measured with an ohmeter?
Any other thoughts for when bi-wiring is better than a single run?
cdc: let your ears, rather than theory, be the judge. In my system, single wiring betters bi, by far.
I've just had to my consideration an unsuccessfull bi-wire.
I don't know when can I stop experimenting but I lost the details mixing Analysis+ 12 with VanDenHul CV125H. I previousely used only VanDenHul unterminated with wires threaded through the wholes in the binding posts with no jumpers. This setup resulted an incredible transperency and all I needed is a-little larger bass extention but I was unsuccessful applying a different cable into the system.
I can also assume that there might be another setup to try but whatever I've compared doesn't even get closer to my single-wire VanDenHul CV125H.
BTW the VDH I use is not a cable but rather a speaker wire but its performance can smoke JPS Superconductor bi-wire which I've used before...
Theoretical background of that issue has too many variables that you will have to consider before buying. That's why it's much more reasonable to research by listening...

Good Luck!
No, you cannot measure with an ohmmeter. The way to measure this is to put an audio sinewave generator through a small resistor and drive the speaker directly. Then measure the AC voltage drop across the resistor with an Oscope or true RMS voltmeter and calculate the speaker impedance from that voltage.
When you want to give manufacturers and dealers some more of your cash...
Sonically I like bi-wire but some speakers sound better with single wires and jumpers. Need to compare.
Does the crossover makes a difference? Like 1st order Thiels don't allow bi-wire but higher order designs use bi-wire. B&W recommends it for "serious listening" in their owners manual.