While the Unicode currency group does include some obsolete currencies, it also includes other current ones, such as the Nigerian Naira and the Ukrainian Hryvnia, and so is probably the best starting point as to what you include without having to sort out your prejudices as to what should be in or out. But looking at Google's subsetting, I note that Latin-ext adds all the other currency group members (beyond €), and other letters required for all (probably) latin written languages - from Welsh and Polish to Chatino and various un-named African ones... - and so should (?) be included by default if you don't want to be exclusionist. It's all a matter of where you draw the line. (But looking at the font, whether Open Sans includes all Latin-ext characters is unclear)