Depending on how you do Table Selection proper Seat Selection may be more or less important. If you often sit down at tables that are not full Seat Selection may be as important as Table Selection.
Some players have complicated rules for how they chose a seat with different rules for maniacs, LAGs, TAGs, shortstacks etc.
Personally I only have one guiding principle.
Sit with position on the players you will profit most from.
Or in other words, sit to the left of the fish. When doing your table selection you have identified your target fish, when selecting a seat you choose the position that gives you the best possibility to earn as much as possible from the fish.