It's my understanding that the R, TM and C symbols are not required to enforce trademarks / copyrights. So, if you register a trademark and forget to put TM or R next to it, it's not like your content is suddenly fair game. Same goes for C, they look at your history of public use, not the instances of a copyright symbol.

However, those symbols signal to others that a trademark / copyright exists and helps to discourage others from copying it.

INAL