The OP is asking about preferred/select seating (not Europe Select, which is KLM's idea of short haul business class) within Europe, not transatlantic.

As pointed out by NickB, there is no pre-assigned seating in coach on KLM shorthaul. However, elites do get automatically assigned at the front of the coach cabin.

One way to get the best seats is to check which 737 version is flying that day (info is available here about 1 day in advance), look up the KLM seatmaps, and either do a telephone or internet check-in a day in advance.

The 739 is the only one with a few rows of 2-3 seats, but they are nowadays almost always in front of the curtain.

On all other 737 versions, the greyed area of 3-3 seats supposedly has an extra 2" of legroom, but seats are the same. The 738 and 733 have some nice exit row seats, but the 734 does not.

No need to sweat on the Fokkers, all seats are the same.