Older printed editions do tend to omit it before words that start with a
consonant, but the older Byzantine and older uncials tend to have it more
often and tend to observe no rules for its use. It appears to me that in
the Hellenistic period there were no rules as to when to use it. Those
rules, it seems to me originated in the 13th to 15th century.
I also did not see Carl's post before I replied. He has some good
information concerning the iota subscript. There are some uncials that
have it and some that do not. It appears in all the minuscules that I have
seen. I would assume then that it was part of the adaptation of the older
Greek cursive script to the minuscule script in the 10th century (900 to
1000 CE) by the Christian scribes.
Carlton Winbery
Fogleman Professor of Religion
Louisiana College
Pineville, LA 71359
winberyc@popalex1.linknet.net
winbery@andria.lacollege.edu