Should I deny ICMP 3 packets?

I noticed from my logs that my output chain had DENYed a "destination unreachable" packet from my workstation to my ISP's Domain Name Server. Does it do any harm to my ISP to deny this? (I don't need to waste their resources.) Why does SuSEfirewall deny the packet? Why would my kernel want to send it in the first place? dproc