OK, so let us say that I'm flying an approach where I start the stopwatch at the FAF and time myself to the MAP.  In a GA plane I would be inclined to fly the final phase of that approach at either 90 knots or 120 knots groundspeed, exactly, because the chart has those timings.

Now, what if I'm flying a Heavy, and for my particular weight, my VREF +5 is 140 knots, just for the sake of example.  Do I do my own calculation, which is easy enough, or do I do adhere to the next highest speed category, which is 150? 

I guess what I'm asking, are the timings laid out on the chart just an additional convenience to help me know when I'm at the MAP, without having to punch anything into a calculator, or are those numbers more of a restriction, and I must adhere to them by placing myself into one of 5 distinct speed categories (a,b,c,d,e), conservatively using the closest one, rounded up? 

Obviously, by doing so I would be making a go-around decision at an artificial point in space, before I actually reach the MAP, but is that how it might be done?

Yes.

Fly your airplane at its proper final approach airspeed. Add or subtract the headwind or tailwind component. See where that ground speed falls (proportionally) between adjacent values given in the "FAF to MAP" table. Estimate the time based on the values in the "Min:Sec" row of the table.

Art