As a newbie is it appropriate to write custom code or....

[eluser]CroNiX[/eluser]
Depends on your needs and goals. Personally I try to use others (established AND maintained) libraries/extensions so that I don't have to waste my customers money building something that already exists elsewhere. Chances are that their library takes things into consideration that you might not think about. I don't want to spend a few days writing a user authentication system, testing it, making sure it's secure, etc., when I can use one that many people have contributed to and refined with the best security practices and start using in 10 minutes.

But if you're new, it's also not a bad idea to see how others write their libraries to see what they're doing. It will help you learn CI and become a better coder.