<p>To echo others: the comment was not presented as a formal rule, and is not a formal rule. It's itself advice, and good advice so far as it goes, but clearly more could be said. </p>

<p>My own view for the question discussed is that advising people to consider quadratics, let alone higher-order polynomials, is advice somewhere between dubious and misguided. That's naturally a judgment call. Positively, I say that  because fitting exponential decline seems a much, much more natural starting place. Negatively, I say that because quadratics and cubics may be non-monotonic and may predict negative prices and may even do either within the range of the data. That's difficult to defend when better models appear to be better starting places. Indeed, to my eye a quadratic is predicting negative prices within the range of the data. </p>

<p>Otherwise put, in this specific case, quadratics and cubics won't even necessarily solve the perceived problem of negative predictions. </p>

<p>More generally, recurrent issues: </p>

<ol>
<li><p>It may be advisable to tailor advice to the perceived level of the question or questioner. That is always tricky, and not just because it may be hard to estimate. 
(In this specific case, the OP does not seem to grasp exactly what logarithms imply.) 
The stance of the forum is that questions and answers of real value should always be of interest to people other than the OP, much though many posters have interest only in their own immediate question. So, often there is a judgment call on addressing a very specific question directly or trying to broaden it, or even to shift it in more interesting or useful directions. (But advising that the OP needs help from a professional with greater expertise is often the right thing to do.) </p></li>
<li><p>Imparting caution is advisable, almost always. Often a question can be answered at least in part, but a complete answer on what should be done depends on any or all of (a) knowledge of the real goals of the analysis (b) explanation of the underlying scientific or practical context (c) access to a dataset. It is common that any or even all of those are not given adequately to allow a full answer. </p></li>
<li><p>Rewriting the question may be needed. Sometimes the question needs even to be turned around completely if the OP is not to remain wrong or confused. For example, it is common for posters to focus on some detail of significance testing when the larger unstated question implies that the test they are using is inappropriate or some quite different focus is needed. </p></li>
</ol>
