You absolutely want to check receiver voltage after EVERY flight. That is still no guarantee that the battery will not fail in flight but it is about the best you can do to ensure that you're good to go. My receiver battery went south between flights this winter. I had just checked it with the ESV and it was around 4.9 v. I was just lining up for takeoff when my horses got loose in the field I was using and I had to shut down to get them back in the paddock. When I came back ( about 15 minutes) I switched the receiver on and........nothing. The battery had gone dead just sitting there. I don't like to think what would have happened if I had gotten off the ground. After that I installed a back-up battery as CGRetired described for a little extra peace of mind.
As a side note, when I took the bad batteries to Radio Shack for recycling the owner thought that thermal shock might have caused it to fail. Anybody heard of that?

Rufcut