I just spent some time deleting APK's using Astro/Appmanager.
What I found is that every time there is an update to an app the APK's stay there - so you have old APK's that can be deleted to make space.
Also very often when you uninstall something the APK's are still there.
Technically you don't need any of the APK's in there unless you need to re-install. However you definitely don't need older versions.
I deleted a ton and cleared up a ton of space.