Why to declare the city a scout is sending goods at the "Start of the Turn Phase"?
If could affect the game in the way, that you do not know if some player will trade with you. If he does not accept a trade you might be interested in, you could lack the ressources to do what you intended to do and have misplaced your pioneer (or vice versa, could have done some better move due to a surprisingly good deal).

However, we always decide where to use the pioneer during city management, since the game rule causes unneeded memory issues that are only relevant in niche cases (the game is already complex enough by itself).