There are two reasons to use a "true rinse" product.
1) pH correction
2) surfactant residue removal
Since this thread focuses only on the former, one has to consider what kind of pH correction is needed.
Most of the time, for simple restorative cleaning, you are simple wanting to leave the fiber in a favorable pH range, as opposed to an undesired pH range that your prespray might be. This being the case, you are best off simply choosing a rinse product with that desired end state pH. If your goal is neutral, then choosing a neutral rinse product is just fine.
However, if you have a situation where there has been taken place a reaction on the carpet due to high alkalinity, then an acidic product might be required to reverse or improve the situation. This being the case, perhaps a slightly buffered acidic rinse would be preferred. But in all cases, you ought not select any rinse product you intend to use as a final step be one that would be a pH outside what would be favorable to the fiber if left remaining. This precludes the logic that one would use an acid with a pH as far from neutral as the undesired beginning state of the carpet is alkaline past neutral. In other words, it's not a simple task of arithmetic to decide what pH rinse to use. Simply select one with the desired end state pH and thoroughly flush. In those cases where you need an acid for correction, simply try for one that will have a compatible end state pH, and you're done.