A lower vf is not always better. For an example if you plan on using a Li-ion with a peak volt of 4.2v, but rated at 3.7v.
You would not want to use a H bin (3.03-3.27v) with a Volt in of 4.2v. if voltage in exceeds vf, then effectively you go into direct drive. In this example where volt in is 3.7 to 4.2v, a better match would be K bin (3.51-3.75) safely factoring for half volt over vf. As voltage will quickly drop to 3.8 range.
In Mr Bulk's SNII direct drive built instruction, he specifies use of a high vf bin to match the 3X 123 used.