Database character fields

Not looking to start a war over which is better, just looking to update my knowledge.

When we create character fields in DB2/400 we usually setup fixed length, no null support fields. If the character fields is long (e.g. 100+ bytes) and we suspect the data will vary greatly in actually used length, we will make it a variable length field. The only time we use null fields is for data we are importing in from other systems where the file can have null values. In those cases the file with null field support is usually a work file used for the import and they we move the data into non-null fields in the production files. In other databases (like MS SQL) the standard looks like it is just the opposite. All character fields are variable length with null support unless you take extra steps to not do that. My training (and it has been some years) said that variable length fields are good for saving storage space but bad for overhead. That the database had to do extra work to manage the variable length, tracking the actually number of bytes in use and manage the overflow areas when the data in the fields changed from 10 characters to 1,000 and back to 10. Is it still true that variable length fields are less efficient and if so why do other databases have that as the default? Or is this something specific to the implementation of the database? Is DB2/400 move efficient with fixed length but MS SQL more efficient with variable length?