Hey Rotten Apples,
I think what your advisor was saying is that TMDSAS dont actually use + or - in calculating your GPA (they do their own calculation of your GPA instead of just taking the GPA your school shows). It's actually a state law. I'm pretty sure one reason is that not every school has + and - so it's not fair for them to give someone who got a 92 in a class 3.66 grade points b/c their school gave them an A- and give another student who got a 90 in that same class the full 4.0 b/c their school doesn't have +/- grading system. Also, some schools that do use the +/- grading system have A+ while other's don't. My University, for example, uses + and - but the highest grade you can get is an A (4.0) while I've heard of other Universities that give A+ which they calculate as 4.33. It's just a way to ensure that the GPA calculations are uniformed.
I think there are some people who are somewhat blind-sided by this b/c it's possible for your GPA to drop some (assuming you have more + grades than -). On the other hand, some might be pleasantly surprised (assuming they have more - grades than +.. their GPA would go up slightly).
Your C- and B+ would actually balance eachother out anyway, so in that scenario, it doesn't matter much. Best of luck to you!