Having sold or installed thousands of tubes in my life (the vast vast majority of which were Kenda or IRC or some other inexpensive brand) I can recall one or two occaisions where I saw an an actual defect in the tube. Plenty of flat or punctured tubes that people thought were defective were actually caused by rough handling, improper installation, or a poor rim strip.
I use whatever tubes I can find that fit, usually the cheapies. I buy 1 or two tubes per year, and usually go through one whole patch kit to keep my family's fleet running.