It's more than a matter of personal pride to produce projects that go through DRC with "No errors found". Ignoring (or overriding) any errors not only becomes a slippery slope of tolerating increasingly questionable situations, but it also becomes an environment where REAL errors can hide among the false alarms.
Power packages with thermal tabs - like SOT-223, TO-220, TO-252, etc - can be troublesome when the thermal tab is internally connected to another pin. If you give the tab the same pin number as the other pin, KiCAD expects to find an explicit electrical connection between the tab and pin (as you describe in this thread). For many of these packages it is a simple matter to run a short trace between the tab and its associated pin. The TO-223 is a little unusual in that the thermal tab doesn't extend very far under the body of the part, leaving a sizable gap that can be used for routing traces, which prevents placing a trace between tab and pin.
If you give the tab its own unique pin number, that number will not be associated with any net. This becomes a problem when you try to increase the tab's thermal performance by adding a copper zone. Of course, you can add a pin for the tab to the schematic symbol and show a net connection, but then you have a non-standard symbol.
P.S. - I will guess that you have done at least a crude power analysis to determine whether you need extra heatsink copper connected to the tab on your SOT-223.