Park it on a flat spot and don't move it.
Seriously, a ROPS won't prevent a rollover - but it might save your lift if the tractor does roll. You can do the following things to help prevent rollover (but they fall in your common sense category as well):
1. Rear wheels set at widest spacing.
2. Rear implement kept as low to the ground as practical
3. Loader (if you have one) kept as low to the ground as practical
4. Drive up and down slopes, not across them
5. If you install a ROPS, you will have to install and use a seatbelt for the best protection.
I have an F16D with no ROPS. I have a Kioti DK 45 with a ROPS. I don't take the Kioti anyplaces that I won't take the F16 and vice versa. Having a ROPS on the tractor does not make me any more likely to see just how steep I can get.
I have mowed some fairly steep slopes sideways with both the Kioti and Yanmar. I think that they Yanmars in general may feel a bit "tippier" because on many of them, the top of the seat is about even with the top of the hood, so the operator sits relatively high on the tractor. I don't think I've ever come close to rolling the tractor, and don't want to. If rolling is a big concern for you, set aside the $600 - $800 and purchase and install a ROPS and seatbelt. If it's not a big concern, just be careful.
Good luck and take care.