Since I haven't found a fabric yet that didn't shrink after it was washed, I wash everything that comes into the house, dry it, fold it and put it away, then iron just before using them.
I use spray sizing most of the time. When I iron the fabric, I always push the iron with the grain of the fabric so there is no stretching going on, and I try to make sure of this when using smaller scraps before ironing. My iron has a water spray feature, but since it has to be refilled often, I prefer to keep a spray bottle handy to help with any deep wrinkles.
When sewing seams, I press as I go, rather than iron. Again, I try to make sure I know where the straight of the grain is to avoid any possible stretching.