TBN into BS is a DPS gain only in the below scenarios:
- if it allows for an extra BS in a dps buff window (e.g. trick attack)
- if the boss is going to be untargetable resulting in your combo to be interrupted
- the boss is about to die

Basically, it’s sometime a minor dps gain, sometime a minir dps loss. For consistency, it’s better to avoid it overall because there’s more risks than benefit.