Decent stereo speakers don't come with speaker cable included by the manufacturer.
Speaker cable really just has one main job to do, and that's to deliver the current demanded by the speakers with minimal loss. It should do that without imparting its own sonic signature on the audio signal. The current and voltages at play can be large. As long as the amp's power supply and design is proper, then 20W/ch of power in to an 8 Ohm load will result in a current flow of around 1.5 Amps.
The fact that the current flows and voltages are high compared with line level signals is the reason why speaker cables are not generally shielded. There's no point. The signal flows in the wires obliterate the much weaker induced flows from electromechanical and electromagnetic sources of interference. To put it in perspective, it's like worrying that one fan clapping and whooping will ruin the amplified sound for a concert in the O2 arena. That's why speaker cable is not routinely shielded.
The amount of current flow through a speaker cable can be adversely affected by the cable thickness (gauge / cross sectional area 'CSA'), length, construction and material. Copper is the optimum conductor material. For loud speaker use, there's a sweet spot between the CSA and the number of individual filaments: cables with hundreds of very thin filaments are bad for power transfer because the single filament CSA is too small. Too few filaments tends to coarsen the treble response.
The thickness required goes hand-in-hand with the cable length and the amount of power it is required to carry. For a high power amp (100W RMS in to 8 Ohm @ 0.015% THD, 20Hz-20kHz, continuous) and say 10 metres of cable per channel, then a 2.5mm CSA cable would be plenty thick enough to get a decent ratio of cost/performance in power transfer. A 0.75mm CSA cable would throttle peak power.
However, if the amp was outputting no more than 20W/ch RMS at peak then there'd be very little measurable and perceptible difference between a 10mtr 0.75mm CSA cable and anything thicker for the 1.5 Amps being drawn by an 8 Ohm load. Where things become more complicated is that speakers are not a consistent load. Their impedance dips and peaks with frequency. This changes the amount of current that the speaker is trying to draw. NOW speaker cable thickness becomes far more important, but it plays second fiddle to the amplifier's ability to supply current. Everything has a hierarchy of importance.
At a sustained dip in impedance down to 3 Ohms, the speaker is trying to draw around 52W from an amp capable of delivering 20W. The current flow would be approx 4.2 Amps. A 0.75mm CSA cable would throttle the power delivery to a shade over 40W, but not before the amp throws up the white flag and runs out of steam. This is why the power supply in these small Chinese Class T/Class D amps is the limiting factor to their power delivery.
In the main, thicker cable won't hurt, but if the amp can't keep up with what the speaker demands then how thick the speaker cable is could be a moot point. Get the power side sorted first. For everyday use a 1.5mm CSA cable is sufficient for most people with a living room stereo system.
Bi-wiring.... Worth it?
I think it depends on the speaker and the listener's preference for the sort of sound they're looking for.
Firstly, I think bi-wiring started out with the right intentions, but then a lot of #metoo manufacturers followed the trend and suddenly people had to have bi-wire terminals without really understanding why. It's a sort of Emperor's new clothes feature and a useful marketing tool to add perceived value to a product for very little cost and so boost profits.
Secondly, the sort of speakers that I like tend to focus on time alignment of the drivers. Bi-wiring often upsets that.
Some others have suggested that the very same brass jumpers that enable bi-wiring in the first place actually contribute to a worsening of the sound compared to single terminals on speakers. One step forward, two steps back. My JM Lab (Focal) speakers have bi-wire terminals. I unscrewed the binding post plate and combined the HF and LF feeds, so effectively reverting them to mono wired speakers. Interestingly, the brand has reverted to mono wiring on many of its high-end speakers. It's their view that bi-wiring often messes up more than it improves.
My own experience with bi-wiring is that it creates a sonic hole between the midrange and treble. The sound becomes disjointed.
I think there are bigger priorities with speakers than worrying about bi-wiring. #1 Get some rigid support that allows the drivers to do their job without the speaker energising what they're sat on. #2 Position the speakers correctly so that there's not too much bass overhang from being too close to the back and side walls. #3 Set the width and toe-in so that the speakers focus and image properly.
