There are two factors that go into determining gutter position on the eave, the pitch of the gutter and the roof slope.
Gutter should be pitched 1/16″ or greater per foot for proper drainage. Gutter can be run level for appearance but addition downspout will be necessary
Gutters should be placed below the extended roof line so that snow and ice can slide clear.  Steeper pitches require less clearance.

Required Clearance of Gutter Below Extended Roof Line

 Roof Pitch                              Clearance Below Extended Roof Line

0 in 12 – 2 in 12                                            1”

3 in 12 – 5 in 12                                            ¾”

6 in 12 – 8 in 12                                            ½”

9 in 12 or Greater                                          ¼”