Biwiring not only non-essential
it's a complete waste of time and money. All you are doing with bi-wiring is in effect pulling the jumper from between speaker posts back to the amp and using twice as much wire as a result. No qualitative improvement has ever been scientifically proven.
Bi-amping, however, is beneficial, wheather you're doing simple bass management (which relieves the main amp from pushing bass singals and the main speakers from trying to produce them, yeilding better dynamics and higher overall output), or going fully active (i.e. active crossovers, direct coupling of amps to speaker sans passive crossovers, which allows amp optimization for each driver and eliminates the phase problems inherent in passive crossover components).
Regarding wiring, I only use heavy guage, cheap *** wire that comes off a giant spool at the local electronics wholesaler (I go to JD Saunders in Boulder, out on 47th). Keep wiring expenses to a minimum, but don't skimp on guage if you have long runs.