The cone should move forward.
It doesn't matter. Whether the speaker starts off one direction or the other won't matter. The sound wave is a displacement of air pressure created by the movement of the speaker cone. Whether you move in and out, or out and in, it's the same displacement of air. Now, it can matter if you have the same or a related sound coming out of a different speaker, such as a subwoofer or P.A. system. Then you have the potential for phase issues. At certain distances, however, it may sound better to intentionally place two speakers out of phase with each other so that the difference in phase correlates to the distance between the two speakers and aligns the peaks of certain desirable frequencies up to be in phase with each other to give a pleasing boost in and near those particular frequencies. But don't worry about that. As long as all of your guitar speakers are close together and in phase with each other, it'll sound the same no matter which way you have them wired.
Case in point. If a speaker out of phase actually sounded worse, then why do Fender amps have their reverb channels 180º out of phase?
Also, a sound wave can start off pulling a speaker in, just as easily as it can pushing the speaker out. If you record a guitar amp into a DAW and zoom into the wavefile, you can see that some sounds may start off above the axis (pushing the speaker out) and other sounds may start off below the axis (pulling the speaker in). You can't hear these differences, but you can see them. And who cares what you're amp's sound looks like. It only matters what your amp's sound sounds like.
3) If you play upstrokes instead of downstrokes (or play downstrokes on a guitar that's flipped over Hendrix style) the phase of the audio signal flips 180 degrees and therefore the relative polarity of speaker inverts.
If you have a junk 2-button footswitch and a soldering iron, it's easy enough to wire it up to reverse the phase of your speaker, so you can hear the difference without a time delay between the samples, and judge for yourself.
The problem is, when you consider absolute phase, you have to consider everything in the signal path, including your guitar pickups.
So why then, when I pluck up or down does the wave form begin with either a positive or negative peak respectively? This is very easy to demonstrate for yourself if you have a way of recording on your computer.If I learned only one thing from the thread I quoted, it is that this is not true. It is the magnetic field in the plane perpendicular to the plane of the pickup that controls polarity of the signal. Up and down strokes produce the same motion in the plane perpendicular to the plane of the pickup. To reverse the polarity of the signal from a normally plucked string you have to pull the string up, away from the pickup, then release it.
There's no way you can hear that and there's no way it makes any difference. With higher notes, the difference is even smaller.