Inverters tend to raise the chance of having a noisy system because of the fact that they put high voltage ac in close proximity to everything. The actual way that the noise gets to the speakers is that electromagnetic interference is radiated from the ac devices and wires and somehow finds it's way to audio wires. If the audio wires are too close, or if they have ****ty shielding, or if they are improperly grounded, they are more likely to pick up that interference and carry it to the speakers. Dc-dc setups also produce emi, but it is a fact that there will be more emi present around an inverter setup than a dc-dc setup. What it all comes down to is just how much of that emi is finding it's way into your audio cables.
Folks who say pure sine wave inverters are better for carpcs are saying that because they saw that someone else said it. That someone said it because he heard someone else say it.... I don't know exactly who first said it, but they're not entirely right.
Folks who blame their noisy setup on their inverter need to learn a bit more in the area of keeping emi out of their audio cables.