I'd go for the option of having a banner that reminds the user. By "banner" I mean a bar at the top like that one that appears to tell people that the site is about to go into read only mode for maintenance, or that a question one is composing an answer to has been closed. (I think that's what you mean by "banner," too.) This banner should include a link to the tour page and it should not appear constantly, nor in a way that is annoying to users. It should be easy to dismiss this banner and immediately obvious how to do so. The goal should be to get readers to take the tour the moment they are actually going to pay attention to what it says and not a moment sooner. Doing this wrong is worse than not doing it at all.
If we force people to scroll to the bottom of the tour before they can use the site, then the tour will be like one of those license agreements you have to scroll through to click Accept. By this I don't just mean that people won't really read it because they just want to move forward to use the site, but also that people will interpret the requirement to complete the tour as a meaningless hurdle imposed by a whole bunch of aloof folks in suits in a faraway boardroom.
Since it is possible to post user-contributed content on the site without registering--this is at least permitted for answers--making the tour part of the registration process would fail to reach many of the people who would most stand to benefit from it. It is also possible to take the tour before registering. At minimum, users who have already completed the tour before registering should not be forced, or even automatically suggested, to do so again. That should be easy to prevent because when one completes the tour one is awarded the Informed badge, which already carries over through registration.
Whatever we do, under no circumstances should the tour reminder take the form of a funny red dot appearing randomly atop an incomprehensible stapler icon.
