The answer lies in your cutting on the pattern line vs to one side vs to the other side... and "correct" could depend on the pattern maker. If you were to cut the slots first, then outlined them on the tab pattern (to modify the pattern to what you actually cut), keep in mind that the entire outline you draw needs to be kept as the outer border of the tab. Or you could cut tabs first and outline them and then the line needs to be kept as the border of the slot.
Gary MacKay has published many projects that use the technique and he describes how to do it right for his projects in his magazine articles and box-making book. My recollection is that he says to cut the first one on the line to remove the line, and for the mating part to keep the line as the border of that part.
Bottom line ( pun is intended!) is that you don't cut both parts on the pattern line.
Spence
PS In re-reading your question, I now wonder if you wanted the info I gave for cutting tabs/slots, or if you are talking about desire to create patterns... perhaps the answer to that is that the dimensions of tabs should exactly match the slots and in use, the cutter needs to follow the above procedure.