I use them in between playthroughs and in between farming opportunities when I start to feel my equipment lacking, if not I save them. I don't use them with max level characters. I may also use them when I'm bored because I have 30 or so saved up which is common around these parts.
Use them when you need them. Don't worry about the optimal level to use one. If you're at a point in a playthrough where you're saying "damn, I need some better gear", no matter what level you are, that's when you use one.
There will be more shift codes, so don't fret over using what you have.