You need a 'single frame reverse' button in addition to your 'single frame forward' button.

When trying to locate the exact point (frame) to start a cut or end a cut (e.g., for removing a commercial or unwanted scene) you need to add the convenience of a backward frame-by-frame movement as well as your forward frame-by-frame button. This would make locating an exact frame much easier (as it is normal to overshoot the point the first time around).