I would change your "many" to "almost all". Very few websites, even sites that really should know much better (like Newegg or Gawker, both catering to technical audiences), actually implement things in a known safe manner. Many just do plaintext (like Newegg) and many an archaic and/or useless obfuscation mechanism (like Gawker's use of DES).
You should always act defensively when relying on any third-party. Very few are trustworthy. Use a reliable password management system for your passwords (like KeePass or mailing GPG-crypted passwords to yourself) and generate a unique pass for each site. Use a unique mail if you can. If you're storing any data on there, make sure it is encrypted and triple encrypted.
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I would change your "many" to "almost all". Very few websites, even sites that really should know much better (like Newegg or Gawker, both catering to technical audiences), actually implement things in a known safe manner. Many just do plaintext (like Newegg) and many an archaic and/or useless obfuscation mechanism (like Gawker's use of DES).
You should always act defensively when relying on any third-party. Very few are trustworthy. Use a reliable password management system for your passwords (like KeePass or mailing GPG-crypted passwords to yourself) and generate a unique pass for each site. Use a unique mail if you can. If you're storing any data on there, make sure it is encrypted and triple encrypted.