Normally I would start filling in the less attractive coastal city sites after CS allows chain irrigation and courthouses allow lower maintenance costs when the number of cities grows. For me any coastal site that can have 5 or more food is worth settling. However I treat these cities differently from you. They are not sources of commerce but cities for whipping and drafting, so I don't build a lot of economic infrastructure, no MGB and only a university if it's needed for Oxford. In fairness I only regard the coastal tiles as a way to grow the city between whips and the commerce is used to break even, I don't work them intentionally. If I was Financial and had the Colossus then they might be considered a source of commerce but without those benefits they are just a convenient working medium for the growth needed to support whipping. In my last game I had an odd area with no resources, 4 grasslands and about 8 coastal tiles. I farmed the grassland (when many would have built 4 cottages) and whipped in granary, lighthouse and courthouse, followed by forge, barracks and eventually drydock. Whipping this city once every 10 turns meant I had a steady supply of units, 60 food in 10 turns is enough for a maceman or cannon and a draft. This carried on for about 200 turns, so this city produced about 10% of my military. The other advantage of settling these marginal cities is the increase in population which helps with votes. At the end of the game the city was able to whip out a battleship about once every 10 turns [drydock, PS, SP, forge and Kremlin] Not bad for some junk tiles that many would not settle.