I'd say it depends on the size of the 12V network and the number of transformers and their size. The problem is two-fold.
First of all, we have to think about the conversion losses incurred. If (and that's a big If) the efficiency is the same on the bulbs than it would be on a central transformer, then the losses incurred by having a central transformer are on par with the one incurred by the distributed light bulbs. However, usually a transformer has an efficiency peak (for example, if the peak power of the transformer is 500W, the efficiency peak could be at 460W. When the system uses more or less power, the transformer is less efficient). This has to be taken into account when calculation potential losses.
Secondly, there's a wiring problem. When you route 220V or 110V, the cable section takes into account the maximal current that will have to be transported. In France, you can route lighting wiring on 1.5mm² cable (AWG15 or 16). Using LEDs will reduce the power used, but yet, as you reduced the voltage, the voltage drop may increase if you use a cable too thin. For example, if you use 12W bulbs, they will draw 1A of current at 12V instead of 0.05A at 220V. A cable too thin may incur a voltage drop too important that will prevent the LEDs from lighting up.