In my ETOPS experience it was 'required' to be done towards the end of the ETOPS sector which satisfies the 'next sector' ETOPS requirement, but if the a/c has come off an non-ETOPS sector you would have no idea until the engine failed.....
Having found 2 x-feed valves actually seized in BA 737's in my time, I always checked it EVERY flight pre-flight anyway.