Notes:
This sets it as a cookie. If you're logged in, this also updates your
logged in preferences and this cookie is then set/updated every time you log in. If you log out, this cookie remains, but it only has your currency and no tracking information.
Having this set will hide extra currencies when you view a listing. You, of course, will need to refresh any listing pages you currently have open for the change to take effect.
If you delete/disable cookies and don't refresh a listing page to have the currency select combobox appear, then it will remain hidden and then whatever currency selected in the hidden combobox will be sent as the currency of your offer. This will either be a default currency or if you selected another currency else at some point in that same window, your browser may keep things at what you last selected.