In the rulebook of Civilization (p.26) it says, that
The player declares which of their cities the scout is "sending" the square to during the Start of Turn Phase, and all the icons in that square (including trade, production, culture, coins and resources) are then considered to in that city's outskirts for the rest of the turn.
In many cases in our play groups we forget to declare the cities at each 'Start of Turn Phase'. Then, we declare the cities at the 'City management Phase'. This is sometimes even an agreed rule, because it makes the 'Start of Turn Phase' a much faster. That is because then you will not have to plan the whole round beforehand at the 'Start of Turn Phase'.
Is there any reason that the decision for the "home city" of the scout should be decided at the 'Start of Turn Phase' and not at the 'City Management Phase'? Would it harm the game somehow, if the decisions are made at the 'City Management Phase'?