How common is it to use the present tense when speaking of deceased persons?
If you describe actions of the person, you would always use the past tense. E.g.
A. A. Milne wrote Winnie the Pooh.
But relationships can be described in either the past or present tense. The relationship between a parent and child is eternal -- A. A. Milne was and always will be Christopher RObin Milne's father.
Similarly, you can say
A. A. Milne is the author of Winnie the Pooh.
No one else became the author when he died, so he's still the author.
I think the choice in these cases is simply personal style. Past tense sounds more natural to me.