I can't set permissions to 775, so is my site insecure?

[thesheep]

The installation instructions suggest setting the various directories (such as tmp/cache, etc) to 775 instead of 777 so that they will be secure.  I understand that setting directories to 777 means that a third party could upload a file and use it to run a Phishing scam, etc.

I can get the CMS to install if I set the permissions to 777.  But if I set them to 775 then it won't work.  I guess because PHP or Apache is running as 'Other' instead of owner or group.

This is shared hosting so I can't do too much about the server configuration.

Is there anything I can do to make the site secure?

Thanks.