If you cut off the tab on only the positive/right side in the Tx, then this forces the servo lead connector to be offset to the right. If it up the correct way then the Negative and Positive will be correctly aligned with the spare Signal contact sticking over the edge where the tab was cut off.
If placed upside down, then the Signal contact will be on the Negative terminal and the Positive will still be on the positive terminal, so no harm will be done by getting the connector the wrong way around, it just won't work.

So trick is to not remove the tab on the left/negative terminal.

I hope my logic is sound.

But then what if you wired the servo lead as Neg, Pos, Neg? Then it would not matter which way you plugged it in as it would work both ways (making sure that the tab on the Tx left/Negative side stays intact.