How much does 96% vs 99% sRGB matter for a photo-editing monitor?

When talking about the extent of a color gamut, all that's really talking about is the most highly saturated bright colours it can portray, which is only one small aspect of colour accuracy.

It says nothing, for example, about the monitor's accuracy in displaying whites and greys, its accuracy in gamma and its ability to portray skin tones, and other mild but important colours, or its contrast range.

96% and 99% are both close enough to 100% that the difference will be meaningless compared to other aspects of colour accuracy.

This is a CIE diagram representing colour as the human eye sees, only the brightest at each colour point (so we're not seeing darker colours here). The vast majority of colours around us are concentrated in a small area around that white point in the middle. The white triangle around it represents sRGB. Circled are the reddest, greenest, and bluest colours sRGB is able to reproduce. In this diagram, all colours outside that triangle are fake, because they extend outside the gamut of colours that the image - and the web - itself can reproduce. As you can see, there are a lot of greens and blue-greens that extend outside sRGB, and these actual colours, which in nature these can be seen in things like butterfly wings etc, cannot be reproduced on an sRGB monitor or image, so the colours you're seeing there outside the triangle are not the real colours. But I digress.

Any RGB colour space will be represented as a triangle in this space between a red point, green point and blue point. The percentage of sRGB coverage will be calculated from the area of that triangle that falls inside sRGB, compared to the area of sRGB itself. So as you can imagine, a triangle that covers 96% the area of that sRGB triangle will not look different enough to be significant.