Cardiologist wants me to take metoprolol ER 1/2 25 mgs daily. Why? Sinus tachy not every nite or day -Blood test/ekg ok holter Minor skip

Your call, I guess. There may be something he/she picked up that you didn't know to share, or he/she may simply be seeking to avoid liability if you have a major rhythm problem. Do you have symptoms that trouble you? Are you hypertensive? Why were you seen? It sounds like a frank chat is in order. Often the best treatment is "nothing" and ultimately it is your choice.