The TYPE of horizontal to vertical connection is what determines whether the combo or a Yand 1/8 bend can be used. In either case, if the horizontal line is a "trap branch" without an intevening vent, it REQUIRES the sanitary tee or fixture fitting, otherwise you can use any fitting you want to. Note #2 of the UPC and Note "C" for the IPC cover this. THe codes are a "minimum standard", but you can exceed them if desired, as long as you do not violate a "mechanical limitation" for its use.
Here is a nice link to Bert Polk's plumbing tips