Consider a simple RC circuit operating in the DC regime. Depending on the specifics of the experimental setup, there can often be a significant amount of parasitic (or "stray") capacitance. In every undergraduate-level lab I've seen on the topic, the assumption usually needs to be made that the parasitic capacitance is in parallel with the "main" capacitor in the RC circuit.
I recognize that this makes the math easier, but I want to understand more fundamentally why this assumption can be made. (OR is it not a great assumption?)
I note in this question about the parasitic capacitance of an inductor, the answer given basically says, "if the capacitance were in series, the circuit would just act like an open circuit." I'm not sure I can wrap my head around whether this explanation helps in my case. In fact, an RC circuit is and open circuit, so I don't really understand why it would be a problem to place a second hypothetical capacitor in series.
What's the deal with this assumption? Is it a good/defensible assumption in many circumstances, or is it mostly done to make the math easy?