My MG gets a dose of it at every fill up. That's partially because the system is completely open, and partially because at a given fill up it's hard to say if I'm going to fill it up again in two days or if the next day I'm going to wake up and start to work on a project that ends up taking 3 months. Under ordinary circumstances, I average 1 fill up this month, but with nice weather and nothing else today, it was more toward every couple of days until it went into the shop earlier this week(finally installing an O/D and maybe rings+cam).
New cars-they're sealed up well enough and are also fairly tolerant of less than great fuel. If I knew I was going to park one for a couple of months, I might give it a shot but otherwise I wouldn't. These days, with gas cheap, I do usually top them off once a week, which both keeps the tanks mostly full and also makes sure that they are at least getting some fresh regularly.