I agree with you wholeheartedly about doing things right, however, topping off when for mixing a developer like Pyrocat, PMK, or Jay's 510 Pyro, is simply not needed. What is important is consistency.
If you standardize on, say, 500ml water (carefully measured out) with (for me) 10ml PMK solution B and 5 ml PMK solution A (also precisely measured out with syringes) to make 515ml of working solution, and if you test with this mix and calibrate EI and developing times to this mix, then, indeed, you have no need to top off and, as Jay observes, you have removed a step from the mixing. Just always do the same thing when mixing.
I, and a whole lot of other careful photographers, mix developers this way, and I think the OP is safe with that method as well as long as he is consistent. On the other hand, if he chooses to top off, and remains consistent with that, then he'll be fine too.
And, BTW, it is necessary to top off when making percentage solutions, since the dry chemical you add to the water will increase its volume somewhat. If you start with the final volume, you'll end up with too much after adding the chemical...