Well, it all depends on you and the types of plants that you want in your tank. Co2 always helps as it's one of the things that plants need to photosynthesize (besides light and water). Also, some plants don't do well unless there are Co2 injections in the tank.
I only use seachem flourish so I can only say so much. Honestly, didn't really need to dose my plants but it really helps them grow a lot better and faster than they did when I didn't use anything. I would say that if you feel that you're plants aren't growing to their full potential. Then it might be time for you to use ferts.
Still a newb, so that's the best I got LOL