Do some simple math.

If you shoot only Fine quality JPEG on an A720IS, it estimates around 1800 files.
Assuming the worst case that only one byte in each 64kb is written to, an absolute maximum amount of wasted space is around ~110MB, or about 3% of formatted space.
In practice, you won't see nearly this high. If you shoot RAW, or shoot higher quality like Superfine, you'll have less files, and less wasted space.
3%? It's not really an issue for camera use patterns.