I press as I go. It has to be done anyway. Sooner or later. My LA quilter's wife emailed me after I dropped off my last flimsy. Her comment was that they always enjoy quilting for me because the flimsy comes in well pressed, strings cut off the back and they don't have to do any prep before putting it on their machine. I took that as a compliment and wondered what other people turn it.
So it comes to sooner or later...and a good end product. Each person has to decide which way makes them happiest.