From victim (which can be end-victim or agent-victim for attack amplification) point of view it is good to block all scanning activities which might detect specific vulnerabilities, patch software, have latest antivirus software, use IDS able to detect anomalous behavior and use patterns. This is because vulnerabilities can be exploited by DoS or DDoS, especially for Recruiting the Vulnerable Machines for agents in DDoS attack, see link: http://www.cisco.com/web/about/ac123/ac147/archived_issues/ipj_7-4/dos_attacks.html

You need to consider DNS as well