Do you feel the ectopic beats? My test showed that I was getting 341 of them, I feel them all. I can't stand it. I'm on Toprol XL 50mg and it has cut the ectopic in about half. I still hate that I can feel them.
P.S. If your HR gets too low you will know it because you'll get other symptoms, e.g., dizziness, vision problems, sweating... lots of non-fatal symptoms to warn you of shortage of oxygen to your system.
Given the average HR of 68, normal or high BP assumed, and the low dose level of Toprol XL I think you should be fine. Check you HR a few times a day until you feel comfortable.
If you have an athletic history, a low HR, even in the 50s may be all you need due to a strong/athletic cardiovascular system.