I recall that deleting some of the pre-installed stuff, specifically themes, could cause all manner of havoc on your device. I don't know if this is still the case, but removing the pre-installed stuff was perilous.

If you're low on memory there are other things to look at before messing with the relatively small files that are the pre-installed ringtones and images. Memory was more critical on some of the older legacy models that only had an internal 16MB total limit. We're talking about models like the 7230, etc.