I am sure this has been asked and debated a 100 times before, and I have searched for it and haven't found an answer....does smbc have to be kept refrigerated after it's put on the cake/cupcakes?

If anyone can give me their opinion on this, I'd really appreciate it!!

You probably won't find a concrete answer on this one.
SMBC/IMBC are on the list of "high risk" products and are banned from a cottage industry kitchen.
Then there are those cakers with a proper (read anything goes) kitchen who won't use anything else.
It is between you and yourself.
If you can be absolutely certain that you can turn out a safe product every single time, I say go for it, it is certainly one of my favorites.