There are of course many factors you should take into account. Placing them 6 tiles apart is one such consideration, but keep in mind that the need for "all" workable tiles won't come into play until much later in the game. Also, tiles with low yield won't be ever worked anyway.
Other considerations:
-Next to river or not. Including defense bonus against melee units. Gardens and hydroplants.
-Next to a mountain. Being able to build observatory and some cool wonders depends on this (or being 1 tile from a mountain)
-Luxury, production, food resources.
-What pantheon and religion are you planning on founding? What culture tree are you planning on opening?
-Jungle. Good for science.
-Defense. Jungle? Mountains? Marsh? Hills?
-Distance from the capital. Both important when building roads or defending.
-Distance from AIs. Important for diplomacy and defense.
-Overall space limitations. Sometimes, when you lack places on the map to found cities.
-Coastal or not.