Memory quilt
 

I have started cutting up shirts that were my dad's. Some of them are more of a seer sucker fabric, with diagonal stretch.

Should I use starch when I cut these? I have never used to start before so I'm not sure what would be best.

Starch would be good, yes. but I would highly recommend some kind of iron on stabilizer as well.

A lot of us us Shape Flex fusible stabilizer on the back of shirts for memory quilts or T-shirt quilts.

I would use an iron on light stabilizer and then cut.

I just finished a Memory (T-shirt) quilt and I also used the light stabilizer - no stretching.

Use an iron on stabilizer with any knits.

I would use a stabilizer, iron preferred, to the lighter fabrics. You want to keep all your fabrics nice and firm.