Most caches can be cleared safely. The only noticable issue is usually a slowdown or two as the program or service rebuilds the caches you've removed.
The iTunes artwork cache is in the username/Music/iTunes/Album Artwork/Cache folder, and can be removed manually. I'd recommend you quit iTunes before doing this.
You do realize that cache is there for a reason. It is usually data that will be needed again so it is kept in cache where it can be accessed faster when it is needed again.
Yes. Some time cache does get corrupted. When that happens the cache must be cleared. That happens far less frequently then people clear cache.
Generally all that clearing cache does is slow your system down until the cache can be refilling again.