If time tracking is activated in JIRA and a user has the rights to browse a certain project, there is no way in JIRA to restrict access to time tracking information, i.e. time logged with all description details as well as work estimates will be visible to such a user. Even though you can restrict the right to log time, you can't prevent users from seeing all this information. This might be perfectly okay within a developper team, but probably is generating huge problems if you grant your customers access to a project. There is no proper solution for this as JIRA does not offer this level of permissions.
If you want to dive deeper into the question of restricting access to time tracking on "field level", you might want to have a look at the following topics: http://jira.atlassian.com/browse/JRA-2364 and http://jira.atlassian.com/browse/JRA-27613. There is also a plugin that partly solves the issue (https://plugins.atlassian.com/23216), but this requires a JIRA patch and still there is a way to get time tracking information if you know how.
As we think that it is essential that you decide whether your clients can see time tracking information or not, we have added a small workaround to ictime that helps you to superficially hide this information from your customers. You have the following configuration options:
Our solution is JavaScript-based and only manipulates display of the data. Any user with very basic programming knowledge will easily be able to get access to the information. In addition, there might be other places where time tracking information appear and that we still did not find and could not apply our script (feedback is welcome).
However, as JIRA does not offer a proper solution, we consider this still much better than simply displaying all data. Most customers have neither knowledge nor time or motivation to hack JIRA, so for many cases, this will at least help you to improve the situation.
The Activity Stream gadget for dashboards will also display time tracking information. As there is no way from preventing users with browse project rights to create own dashboards and to add any gadget they want, you can only take away this gadget from the system dashboard and hope that your clients have better things to do than adding new dashboards with the Activity Stream Gadget. We are looking for a solution for this, too.