They can have two; one submitted by you, and another suggested by other users (who have completed at least one hubwheel). Tags are a bit of a contentious issue on Hubski. One pervasive sentiment is that tags are not to be emphasised over users.
While I get that users should be the focus (I've gotten to know a few folks pretty well here), It's helpful for things that fall under multiple topics. Where do you tag your post about John Williams writing music for the new Star Wars movie? #movies? #music? I do think waiting until they fill out a hubwheel is a good idea. I'm more than halfway into my first one, and it's nice to be able to track how much I've contributed.
#starwars? Try to think about why you care about it. It doesn't have to be too general.