As long as you don't do something really stupid, like I once did when I was on coreg, I think you're OK. I noticed that I had skipped the morning's pill when I went to take my evening pill. So I took BOTH pills at the same time .... arrrrrrghhhh. It says right in the insert, don't do that. After a bit of panic I called the advice nurse who asked me a few questions about BP, heart rate, etc. and told me what to look for to see if I needed to go to ER. Luckily,I never had to & there were no repercussions. The standard instructions are if you miss a dose, & don't realize it until you are near the time of the following dose, just don't take it. One skipped pill isn't going to do that much harm.
As for getting off beta blockers after valve surgery, I was told from the beginning by my cardio that wouldn't happen. But I had CHF as well as the valve problem. My cardio said that once the valve problem was attended to, thus lessening the burden on the heart muscle, it should be easier for the beta blocker to do its job of (hopefully) remodelling the left ventricle a bit.
Only difference post surgery is that I am now on lopressor (metoprolol) instead of coreg. This makes me a bit nervous because the cardio told me that coreg was "the" best BB for people with CHF. At the hospital the discharge doctor explained the change by saying that coreg is very powerful, with potential side-effects for people just out of surgery. Hence lopressor. I haven't seen my cardio post-op yet, and will have to ask him whether at some point I will go back on coreg.