This still happens and it is so common that companies actually hire firms to search for unexploded ordnance before digging in Germany.

There was also a Fire Balloon that detonated during a picnic in 1945. They also recently found and detonated one in 2014 in British Columbia.

In everything I've read, the number of civilian casualties is usually rounded to the nearest 100 or 1000. Everything seems to be approximated, so adding 1-10 or more names, really wouldn't change the statistics that much. Especially in places like Germany, where estimates are in the hundreds of thousands to millions of civilians.

Usually the word "approximately" appears a lot, and the estimates have a variance of tens of thousands, if not hundreds of thousands of people. Record keeping in that time period wasn't easy, it would get especially difficult in the rural parts of some countries to keep specific records.

Usually when the subject of civilian casualties during World War II comes up in my reading, it's restricted to the dates in which the war was still being fought. To sum it up, even if someone dies due to unexploded ordnance from World War II, statistics aren't going to change noticeably and books aren't going to be re-written. It would seem to be more of a very unfortunate accident rather than a casualty of the war.