Hi Steve,
you are correct in your assumption. Everytime you want to view the Last Access field (e.g. in index dialog - system entries) you have already accessed it. This means, that everytime your username will be visible.
This system field is mostly used for some kind of deletion policies. E.g. if a document is not accessed the last 10 years (together with some other filters) the document can be deleted.
Regards
Tobias