I don't think there is a wrong way to play rotationally, but maybe it takes some time to find the right way for one personally.
I play rotationally with just one save file, a lot of families and a normal life span. Only the household I am currently playing is aging. However, once I finish my rotation and cycle back to the first household, I enable aging for unplayed sims, so that my Townies/NPCs age as well. I disable their aging again, when I leave that household and move to the next. This way, my townies are aging in the same speed as my played sims.
I write down some notes, so I know which Sim is born in which generation and no one accidentally marries the cousin of his/her aunt (but even with notes this almost happened ;) ).
Usually I rotate early in the morning ( 5 - 6 am), so I can start a new day with a new family and no child is missing school.
Apart from that, I have no 'rules'. I try to age my Sims up approximately in the same order as they are born, but I am not very strict, because sometimes I have no interest in playing one family for a while and sometimes I have fun playing another for 'too long', sometimes I even disable aging completely, e.g. when Sims are at University. As long as I can rely on my notes when it comes to relationships and marriage, everything is fine.
The thing I love most about rotational gameplay is the freedom I have, but at the same time knowing every Sim in my world. I can play an evil vampire and a standard suburban family or a crazy cat lady - all in one save. Once a new pack is released, I grab a young adult, disable aging and explore everything. I never had the urge to create a new save file, apart from my building save file.