I have IST as well, and have also had an EP (with ablation to get rid of PVCs). Anyway, after I had the ablation for the PVCs the doc told me to stop the Metoprolol, I was on 50mg once a day.. I stopped it cold turkey (he said I could do it!) and I was completly fine.
As for the IST, we've just discovered that within the past few months and one of the things they said they could do about it was go back on a beta blocker, to which I said "Thanks but no thanks!" ;-) I'm trying the "if I don't mind it doesn't matter" approach and I'm with you in feeling like a lot of it is brought on by stress, not knowing if when it happens if I'm going to be okay etc, that doesn't help matters.. Just the reassurance of knowing it's harmless has done me a lot of good! I'm at a point now where most of the time I don't even notice it, and when I do notice it, I don't get all freaked out about it..
I'm aware of the warnings about suddenly stopping a BB, but my father had gone from 3 x Toprol 50 / day to zero per day when he switched to a CCB and ACEI. No problems whatsoever. That was what the doctor said to do, btw.
If you do not have a blood pressure machine, you probably should purchase one just to put your mind at ease. Do as your doctor instructs by taking half and after a week you could cut the dose in half until you are off. If you do not have heart problems or heart failure and just have tachycardia it should be okay. A pulse of 152 is not all that bad. Mine gets up there sometimes too and I do not take anything. If it goes up to 152 and stays there then that's a different story. You could have an adrenal problems which is harmless (at least that is what doctors tell me as that is what I have). This is where you heart will just start beating fast, just take off and sometimes it lasts an hour, sometimes a day and sometimes two days. Exhausting stuff. Mine was beating 169 beats per minute for an entire month when I was 9 months pregnant night and day (exhausting). Hope you feel much better soon.