Hi Emmet,
I took classes before re-applying. The courses I did were all 400 level medically relevant courses: immuno, virology, bioethics, bacteriology. None of these were repeats, all new courses. I double majored during college and unfortunately didn't have the time to take these, so I made sure to take them during my year off before re-applying.
Universities will allow you to take courses once you earn your BA/BS ... but you'll have to look into postbac or nonmatriculated student programs.
One other option to think about is working for a major university near you. The school that I worked for offers a tuition exemption program which allowed me to take these courses (up to 6 credits/quarter) for free. The downside is that you have to wait until the third day of the quarter to register (so it is very difficult to get into high demand courses) but if you are persistent, you can use this type of free program to your advantage.
It is possible to work full time and take courses but I made sure to only take 1 per quarter. I wanted to put up good numbers, so I studied like crazy and made sure to 4.0 every one of them.
It's hard to say "when do you know that you should not apply" without knowing your stats, like the previous person mentioned ... however try to take a good hard look at your application. Be sure to improve something if you are going to apply again without waiting a year ... bump up the MCAT, add new volunteering/shadowing experience, add new life experience, anything. You might want to consider calling all of the schools that previously rejected you and getting feedback regarding what the major weakness was ... you might be surprised, it could be that your stats are fine, just the interview didn't go so well.
Look at the stats for the schools that you applied to, or will apply to next round to see how your numbers compare. I think it's best to shoot for >3.5 GPA and >30 MCAT. Look at MD applicants ... you can definitely get an acceptance with less than these numbers, but the odds will be stacked against you. If you do have a weak GPA, make sure you have some type of extracurricular sparkle to set yourself apart and make up for that weakness.
If you do decide to take a year off, make the most of it. Attack all of your activities with vigor, and enjoy the time that you have. I was initially leery about the idea of taking time off to really overhaul the application, but now that the application process is over, and I'm med school bound, I think the wait is definitely worth it. You'll grow a ton over the year if you decide to wait ... and it can only help you if you work hard during that year.
Good luck and feel free to PM me if you have further questions.
Cheers! 😎