Absolutely press the interfacing on the tees before you cut out your square - otherwise you have to cut the interfacing as exact as the square - more work and more chance of ironing parts that hang over onto your ironing board. If you do some research, the experts suggest tricot (knit) interfacing applied so the stretch of the interfacing is opposite to the stretch of the knit in the tee. I have tried this as well as regular interfacing ---- the tricot makes the quilt much less stiff.