If they fall under Hinduism, I think the reasonable thing would be to accept the question and encourage answers that provide a well-argued answer.
I don't know the answer in the case of nail-cutting activity on Saturdays, but in general there might be a
- traceable history (X recommends this practice in year Y and provides reason Z),
- accepted theory,
- scientific explanation.
The last point might be rare for a question you already label as superstition (rather than custom), but I have come across the question "why should you not practise yoga (asana) on moon days (full moon or new moon)?". This might well look like superstition and I don't know a scientific explanation, but the theory is that on moon days the body heals not as fast as on other days. However, I do know that several German hospitals do not schedule serious operations (e.g. heart operation) on moon days to lower the risk of unsuccessful operations (and they do this out of experience, rather than superstition).
I don't know whether this example question falls under superstition for you or not, but for someone else such a question might well be superstitious in nature. I think we would miss out on interesting answers if we were not to allow such questions.