Instance level doesnt matter.

Each index should be hand crafted, i.e. each index should be <create index blah blah with(fillfactor = X)>

Indexes on identites, which increase monotocically, should be around 100.

Indexes on ever increasing but non-monotonic columns I normally have hovering between 95 and 90.

Indexes on mostly random columns, like userid, I tend to have between 90 and 80, depending on if it is a quickly growing or heavily modified table.

i.e. fillfactor of 100% on a GUID is fine if the table gets inserted into once or twice a day, but will hurt if it gets millions of inserts per day.

This all depends on how often you do index rebuilds, and how often the particular index is included in that rebuilld, and how important it is to not have it be rebuilt daily.

Can you do online rebuilds?

If an index is rebuilt daily due to fragmentation, and you need the index to not rebuild daily, then change that index's fillfactor to 5% less than its existing fillfactor, and iteratively you could get to a good value.

In summary, it all depends, and a blanket instance setting is not the best thing to look at first.

I feel it should be deprecated.