Teeny tiny question

[melinhad.livejournal.com]

I bought some interfacing for the first time and started using it in bags I've made. My question is this: Should you iron on the interface before the sewing of the bag or after? I've done both. I had an old t-shirt I wanted to make into a bag and had already sewn the bottom. So, I turned it inside out and ironed on the interface. And yesterday I made a simple tote, but ironed on the interface to the unsewn pieces. I'm kind of wondering if there's a right way to do this or is it up to each person to do what they are comfortable with...

BTW, here's a pic of my t-shirt turned bag, so it's not text only: