The only reason the spark plug is angled is so that the spark plug and spark plug cap can be removed.  The angled spark plugs have the electrode located so close to the center of the combustion chamber it does do not effect flame propagation.  Locating the spark plug in another location other than the center of the combustion chamber will effect burn speed and when the combustion process is complete.  We want to complete the combustion process as quickly as possible. 

Detonation is more apt to occur as the time for complete combustion to occur increases.

Remember the basics: the pressure front travels at the local speed of sound in the combustion chamber, the flame speed is controlled by the turbulent intensity and heat travels at the speed of light.  Flame speed is relatively slow as compared to pressure and heat velocity.

The speed of sound is governed by pressure and temperature.  The speed of sound in free air at sea level is approximately 1100 ft per second.  The speed of sound in the exhaust at export opening can be over twice the speed of sound in free air.  The speed of sound in the combustion chamber at peak pressure and temperature can be many times the speed of sound in the exhaust.  Knowing these facts we can see that the pressure distribution on top of the piston is distributed evenly at any time during the combustion process.

Spark plug location has  nothing to do with pressure distribution on the top of the piston unless we are strictly being theoretical.  The instantaneous effect of the pressure distribution on top of the piston would not cause the piston to rock unless the piston was mass-less.   Piston rock is controlled by piston pin offset, offset of the cylinder axis over the crankshaft, connecting rod length, the length of the piston skirts, and piston skirt clearance.