Back to the burn test:
I put the candle either in a large metal cake pan, the concrete laundry tubs in the basement, or a porcelain/cast iron sink - and have an ash tray and a glass of water nearby.
If one has a lot of fabrics to check - it's a good idea to do this outside. The fumes can get to be a bit much - especially from the synthetic fabrics.
Back to purchasing fabrics:
As long as I get what I ask for - and the cut is perpendicular to the fold line - I won't complain. (If I buy something at the deli, for example, I have to pay for every bit I get.)
I have learned, however, to check for grain lines, etc. before purchasing - and then if it is off-grain - decide how much it annoys me and go from there.
I've also learned to let the fabric do what it wants to after washing and drying it. I can attempt to "straighten" it, but it seems to revert back to whatever it did after I washed it.