[Christ]

On most engine designs, the piston is only parallel to force at TDC and BDC [yes, you had those correct] which is where skirt/ring friction come into play, increasing at the beginning of the upward or downward stroke and decreasing at the opposite. Then you have actual engine design specs that can compensate to some extent, but that's a whole other debate.

One instance where this is NOT true is Honda's R18 design [I think that's the one] with the offset crank. Well... it's still partially true, BUT it uses an offset crank so that on the power stroke, the piston is as close to possible as pushing a straight line to make the power stroke's output per millisecond more linear on a graph, which is supposed to result in a smoother power application [I have no idea how it performs in real world testing] - the downside is that the I4 engine ends up more like the slant six Chrysler in days of yore - it wears piston skirts and rings more on one side.

With newer technology, much of this wear is alleviated [and the operating life of the engine as a whole is increased] - but the additional wear is still there, although to a lesser extent than it's closest comparison leaning-bank engine.