when i do a tee shirt quilt first i cut out the pattern/design about 3 inches bigger than i want it to finish ( if it is a 6 inch design, then cut it out 8 to 10 inches bigger, just wack it out, nothing fancy at this stage! ) then i use magic spray sizing and iron that design every which way possible but watch for those sticky gluey designs...they will melt and you will throw out that shirt!!
then, i use Pellon 911 FF fusible Featherweight interfacing. i get it at JoAnns on sale for about a dollar a yard on sale....but remember it is only about 20inches wide, so plan on getting enough. cut off a section of interfacing and iron it to the back/wrong side of your tee shirt design and then you are able to fussy cut out your design....easy peasy!!
i've never had an issue with the stretch of the interfacing....just lucky or maybe it doesn't matter??
jodi