In that case, depends on your level of paranoia or what was on the hard drives... If someone was determined, the data could be recovered from physically smashing the drive quite easily...
But, if there's nothing of real value on there, there's no danger...
Just deleting the files, however, takes someone who knows what they're doing about 5 to 10 seconds to recover.
Another possibility is to open the drive up, remove the platters, throw the rest away. Keep the platters and turn them into some interesting wind chimes...

(And yes, I know people who do that, they're actually very cool wind chimes)
The magnets in the hard drives are fun to play with too... Rare earth EXTREMELY powerful ones... If you play with those, be careful, they can pinch the fire outta your fingers if you're not careful!