Early on while I was learning Spanish, my teachers would always make us use the pronoun 'yo' even if it was redundant:
Yo pienso que...
Yo quiero...
Yo hablo...
etc.
However, a more advanced Spanish book I bought told me only to use redundant pronouns like yo when I wanted to emphasize that I was performing the action (and when clarification of the subject was necessary).
When should I use 'yo' in my sentences in Spanish? Does it make me sound rather self centered if I use it constantly? How and when do native speakers use it?