Squid's log files, especially access.log, contain a record of users' activities and, hence, are subject to privacy concerns. As the Squid administrator, you should take every precaution to keep the log files safe and secure. One of the best ways to do that is limit the number of people who have access to the system on which Squid runs. If that isn't possible, carefully examine the file and directory permissions to make sure they can't be viewed by untrusted or unauthorized users.
You can also help protect your users' privacy by taking advantage of the client_netmask and strip_query_terms directives. The former makes it harder to identify individual users in the access.log; the latter removes URI query terms that may contain personal information. See Section 13.2.4 for more information.
You may also want to develop a policy for keeping old log files. Obviously access.log helps keep users accountable for their activities, but how far back would you ever need to go searching for something? A week? A year? What would you do if presented with a court order to hand over your log files for the last three months?
If you like to keep historical data for a long time, perhaps you can make the log files anonymous or somehow reduce the dataset. If you are interested only in which URIs were accessed, but not by whom, you can extract only that field from access.log. This not only makes the file smaller, it also reduces the risk of a privacy violation. Another technique is to randomize the client IP addresses. In other words, create a filter that maps real IP addresses to fake ones, such that the same real address is always changed to the same fake address. If you are using RFC 1413 identification or HTTP authentication, consider making those fields anonymous as well.