I don't fly the 737NG, so can't comment based on personal experience with the aircraft. However, no company or crew is going to take you to a position which is unsafe in your travel. You may find that some days the crew is flying at lower levels (say for example, FL330), and another higher altitudes (FL390, for example). The difference will depend on what the aircraft is capable of doing on that particular day based on weight and outside air temperature at altitude, and the decision as to what level to take will also depend on traffic, winds aloft, and turbulence.
Temperatues at altitude can vary depending on the weather systems and airmass movements, from very cold to much warmer than standard, and this in turnd has a large bearing on how high the airplane can climb and where the optimum altitude will be for any given weight.
A turbine engine is fairly inefficient at low altitudes, especially at slower speeds. the most efficient operating RPM for a turbine engine is at the upper RPM speeds, typically about 90-95% of it's maximum operating speed. At slower speeds the engine is "lugging," much like what you get trying to go up a hill in your car in the wrong gear...it's just not efficient. The most efficient state for thrust in the jet comes when the forward speed equals the exhaust gas velocity. When the exhaust gas velocity is faster than the forward speed, you have something akin to the slippage you experience when driving a car or truck in mud or snow; the wheel is spinning fast, but not doing a lot of work in moving the car. Same thing with jet exhaust.
For a given weight and temperature, the airplane will have an optimimum altitude when aerodynamic performance is balanced against engine performance to give the best of both worlds, or as best as can be expected. Different companies and different aircraft use differnt profiles, but flight at the optimum altitude is ideal. On longer flights, often as the trip progresses the aircraft will climb, doing so in "steps" each hour or so. The aircraft is climbed to take advantage of the new optimum altitude based on weight and temperature, and often for wind, as well. This may account for the different altitudes you experience on your journeys over the same route. Some days lower, some days higher. Outside air temperature plays a big role in that, as to other factors.
When you're at the optimum altitude, you have adequate margins between your cruise speed and the buffet. You're not really in "coffin corner." Typical airline aircraft today don't experience horrendous mach tuck performance or other such bad habits...most have been designed out of the airplane. Excursions up to Mmo (the red barber pole on the airspeed indicator, previously discussed), or temporary dips in speed, won't cause large concerns. What's usually required is a slight power increase or decrease, and this is often handled without any input from the pilots at all, by the autothrottles as a function of various aircraft systems or devices.
What you're feeling in the passenger compartment isn't buffeting, but the effects of some turbulence, perhaps most often associated with windshear along the jet stream boundary layers. This occurs where fast moving air of the jet conflicts with slow moving air, much like the eddys you see in a stream where the faster moving water passes slower moving water or rocks. The little whirls on the surface of the water are akin to the same thing that causes the bumps you feel.
You may notice that those bumps are primarily vertical...you're being bounced up and down. Rather than other types of turbulent behavior (such as dutch rolling tendencies or any fishtailing or shaking), the vertical bumps are really more of the wings flexing slightly and damping out the turbulence.
It may be well to remember also that in the event a momentary windshear condition causes a loss of airspeed, the airplane still has mass, momentum, and inertia...it's not going to simply stop flying and fall out of the sky. While airspeed may change briefly and temporarily, the airplane continues to fly just as it did before, and when the shear condition ends, as it must in short order, the aircraft will be right where it was before with no significant changes. The aircraft energy doesn't really change that much, just a temporary change in local atmospheric conditions and pressures about the airplane, and not enough to cause any significant concern.
You should also remember that with power constant and no trim changes, the airplane is stable in cruise and will attempt to return to it's stable, trimmed condition on it's own, even without asistance from the autopilot, auto throttles, and other devices. The airplane is stable, and wants to stay that way.