Its time for mathfleetgothic to prove and or disprove the theory.
Lets use an evil round number as an example, 18, as the number of WB dice we throw against each necron ship.
Unbraced against all necron ships, 18 WB dice will average 3 hits. A tombship will end up with 1.5 hits after saves. The cruisers will end up with 2 hits after saves, and the escorts will end up with 2.5 hits after saves.
If they brace, they will end up with 9 hits, and 1.5 hits after saves. So statistically we see that the tombship is the same to hurt when braced or not braced, but the other ships will take more damage when not braced as opposed to being braced.
So it does make sense to use WB against them first when they are not braced, and if they brace, try to shoot another target with your lances. Which is a good strategy to reduce the firepower of necrons.
Other than that, which is a good strategy, concentration of ordinance and other firepower to inflict maximum damage possible is good as well. A crippled and braced necron ship won't be shooting back much.