If you are really concerned about security, download they bitaddress.org web site from GitHub (https://github.com/pointbiz/bitaddress.org) and then run it while offline and in private mode on the browser - at minimum. Quit the browser after running it. Maybe even run it from a new user account on the machine. (Or from a USB boot device that is only used off-line.)
There are ways that are even more safe, but it is not recommend to do it online for obvious reasons.
If i make them i always use a Ubuntu Live CD without Internet Connection. Then I am 100% sure know one has see what I made.