Letting them level is a good idea. Also, don't forget Awaken and classing them. Imagine you're companion as a Psion (it's going to happen in our campaign). Also, I don't think dropping your animal friends all the time is very good, they are supposed to be companions and not a block of stats. I do understand that 1HD dog is going to get pounded by the trolls, but a 6 HD mutt, can still be viable...Especially with some magical help from the Druid.
Gariig
Gariig