I wouldn't say a default of 80% is good - nor would I say one of 90% or any other value. There are definitely some tables in your system that should use a default of 0 (100%), while others should be set at a different value.

What that value is going to be should be determined by the activity on that table and not some default value.

If the table is only inserted into and never updated, with a monotonically increasing value as the cluster key - filliing it 100% is valid (and probably the best).

On the other hand, if your table is updated frequently where values change a lot on varchar columns (causing data to be moved) and/or deleted from frequently you would want to set the fillfactor. In these cases, the goal is to reduce how fast the table fragments and how often you have to re-organize or rebuild the indexes.

If your system is not 24x7 and you have the time to rebuild your indexes every night - then a higher fillfactor could be appropriate. Again, depending upon the activity for that table.