Before we see the data. Once we have seen the data it is too late. Actually, the analysis needs to be done even earlier than that, not merely before we see the data but before we collect them. For if we are prepared to do the analysis before seeing the data, we may as well do it before constructing the questionnaire. In that case, we can make certain that the questionnaire will contain everything needed and, incidentally, nothing else.
Traditional practice calls for many measures.
That is judged to be the conservative position. The whole study would be lost. So there is safety in numbers. Suppose we include two measures for a variable, say two different ways of asking the question. If the two measures agree, we don't need both.