Any time. :)
Whether or not cruising stealthily and efficiently at low altitude is included in the design of an aircraft depends on the mission. For something like the B-2, a high-altitude weapons delivery platform, it's not part of the mission.
The reason military aircraft in the 60s through the 90s flew low-level was to fly below the radar, using terrain masking to help hide the aircraft from enemy search radar. These days stealth technology is so good that aircraft can fly at altitude without showing up on radar, so they do.
All aircraft, including reciprocating props, turboprops, turbojets, and turbofans, fly more efficiently at a higher altitude than sea level*, but only to a point, and that point is both an entering argument in the aircraft's design, as well as a result of that design. For example, practical considerations such as FAA regulations requiring the flight crew being on 100% oxygen above FL 430 render flight above the FL 430 fatiguing. Therefore, most airliners are designed such that at average pax/cargo weights, min fuel, and temperatures, FL 430 is the most efficient cruise altitude.
This depends mostly on the weight of the aircraft and the outside air temperature. The weight changes throughout the flight as fuel is burned, and temperature changes with decreasing altitude, but only to a point. At any given weight, there is a most efficient altitude. Flying any higher than that actually decreases per-mile efficiency. A fully-laden airliner, therefore, might start at FL 280, but throughout the flight it will step-climb to successively higher altitudes as it burns fuel and becomes lighter. As it nears it's destination, having burned all that fuel, it might be at FL 420.
* In the rare case of a very heavily-laden aircraft on a hot day, the most efficient cruise altitude can actually be below sea level. In fact, the service ceiling, which is the safest altitude an airplane should fly, can be below sea level! When that happens, it's safer to take the bus.