Back when I was doing my aprenticeship as an aircraft tradesman, I was taught the way to push down the tail of a Cessna or Piper was to push on a rear fuselage frame. On a cessna it is the one at the base of the fin where the forward fin attachment is - All nosewheel cessnas have a removeable inspection panel between the fin and tailplane, and this frame is where the front edge of that panel is fastened. this frame is a machined item and is incredibly strong. Its a simple case of reaching over then fin and pushing with one hand on each side on that frame. Although using a towbar is the preferred method of ground handling, the nosewheel angles really arent tight enough when stacking aircraft into a tight hangar, so pushing down the tail is inevitable. The other reason for doing it is to access the nose gear for servicing. Many engineers will hold the nose in the air by putting a weight on the tailplane. the key thing is to avoid point loading - We protected the tailplane with a piece of carpet, then laid a piece of plywood so it would go over both spars as close in against the fuselage as possible. we then put some weights on the ply to keep the tail down - making sure they were stable and wouldn't fall off. Tieing the tail down is a better option, but not always practical - you often cant get the nosewheel high enough once the tiedown length is taken into account. Tailplanes take that downward load (and more) in flight, but the difference is the point loading.