That's not strictly true. You should cast ES20 even during Ascendance; it deals more damage than LvB even without critting, it only costs a global, and you don't want to be sat at 20 stacks wasting obscene amounts of potential damage.
You can optimise when to pop Ascendance to ensure minimal wastage, but Ascendance is a very weak cooldown, and you will always reach 20 LS stacks during it, so you'll always end up using ES20 at some point during it.