It might be the safest thing to do, but for us in our work flow it is not practical. Event photography with several photographers shooting as much as they can in a very short time window, the need to copy cards and get them back out to the photographers to keep shooting is very critical, especially shooting RAW files the volume of data is huge. Typically the photographer on the line will assume the card was not downloaded when handed a card with images on it, so we end up downloading the same images over and over leading to gigs of additional data. For us it is safer to actually delete the card as it is being ingested to multiple hard drives for backup.
