It depends, of course. Entering Venezuela I needed insurance to get a temporary import permit....and there were plenty of roadblocks to check. French Guiana needed insurance only when leaving. Guyana and Suriname only to enter. In Peru I evaded the roadblocks because I could only buy a year at a time, although more recent reports say you can purchase a month now; in Colombia I used an old American insurance card (not recommended, of course, but it worked for me), while others report heavy fines; in the south, reports vary greatly but people have been forbidden to enter in northern Argentina recently (coming from Bolivia), while I entered and left repeatedly without any insurance. Etc.
You're looking for hard and fast rules. There are none, and if anyone tells you there are you can safely discount anything else they tell you. Everyone's experience varies, and it changes over time as well. Some get lucky; some crumble at first hint of extortion from police. My philosophy is to assume that it will all become apparent at the appropriate time, and if it turns out I need insurance there'll be someone around to tell me so, then sell it to me. It's been working that way all over the world. Sometimes patience is the order of the day....but you're coming from Central America, so you'll be alright on that score.
Hope this is helpful. Have fun!