color film

Let's say you have a roll of Kodak Portra 400 and you shot it at ISO 320 (1/3 stop) or even a full stop at 200 to purposely overexpose it then develop it normally it at its box speed of 400. Would that be a good idea since it's always better to overexpose color negative film when you're unsure...

So a few years ago my father passed and I recently stumbled across his Cannon FTb. I took some photography courses in school and really enjoyed it so I figured I'd give it a shot - I never had used film just digital. I know some what about iso and over/under exposure but it's been a few years so...