I'd say do whatever makes your guests have a good time. Casual playing is for fun, and if they are saying they'd like to have more chances to access more features, there's no harm in giving that to them.
If they become interested in tournaments, and want to practice at your place, that's when I would tell them that most of the time SS games are 3-ball in tournament and you'd want to switch the mode (might also want to make other tournament tweaks as well, like disabling extra ball and such, if they are wanting that sort of experience). But, I don't feel strongly on this, since if the goal is working on basic skills that's going to be improved regardless of ball count (and if your machines are actually the same titles expected in a tournament, then they'll still learn the rules and what to shoot for regardless of settings).
I have 3 EMs and one SS currently. All are 5-ball as that's how I received them, except one EM which was designed for 3-ball (my oldest machine, in fact).