My understanding is the Conductor is required to charge the highest bucket rate.
Back in the day Conductors were allowed to charge the lowest bucket rate.
The best time to check for rate drops is daily in the hopes that the bucket level has changed due to reservation cancellations.
However, since there are only 5 bedrooms (excluding the H and Family rooms) in a Superliner sleeper there are fewer cancelled reservations compared to Roomettes (14 roomettes).