I go soon for an diagnostic mammogram. Will I need a breast ultrasound?

Possibly.... If you had the diagnostic mammogram for something you can feel, like a lump, then you will almost certainly need an ultrasound. If you were called back for calcifcations, you most likely will not need an ultrasound. If a mass was seen on the mammogram, that you cannot feel, you will also need an ultrasound.
Probably. Many times ultrasound is utilized in combination with diagnostic mammography to completely evaluate a potential abnormality.
Perhaps. An ultrasound may be useful to better define a density ("lump") or an area of asymmetry ("shadow") seen on mammography! however, it is of no benefit when evaluating microcalcifications. Ideally, the breast canter where you have your diagnostic mammogram will be able to perform the ultrasound the same day, if they recommend one.
Maybe. If you have microcalcifications, the mammographer may want a magnification view only and no ultrasound. But for most other things (typically masses or significant asymettry) and ultrasound is helpful.
Sometimes. An ultrasound is simply another means of looking at breast (or other soft) tissues. It is not usually ordered routinely in conjunction with a mammogram unless there is something abnormal on the ultrasound or a mass that is palpable.