Along this line of thinking we eventually get to a point of maximum utility for the army book. Where a list is about as effective as it can be at a given points range within the choices available. And then we're in comparative territory that
can speak more eloquently about than I.
You don't need to tailor your CSM list when you have 3 Heldrakes, kids!
But seriously, if you optimise your list, there's very little point in tailoring. That's the point of optimisation. The units in a really good list should always be able to engage something effectively, rather than being bad at fighting a certain army. Ask a Leafblower Guard player if he list tailored. Or a Venomspam player. I'd use Draigowing as an example too, but they don't even have the option to tailor (10 Paladins, 4 MC Psycannons, Apothecary, 2x Hammer, 4x Halberd, Brotherhood Banner. Options: May add Warding Stave).