I have now sewn a few things including a few pillows and a bedcover case and did not (yet) follow this advice:
Always wash and press all fabric before sewing with it. New fabric will usually shrink a certain amount. Marking of patterns must be done on quite flat (well-pressed) fabric, as creases will distort the shape of the chalked pattern. source Wikibooks
What I simply did was marking with an ordinary pencil[^2] on unwashed cotton fabric. Then I began to sew and did not bother much about creases of if I made it straight. I imagine that these advices apply to precision work as is the making of clothes.
^: This might be hard to remove I was told at the fabric store.