I press as I go, saving the trimming until I am ready to put the quilt together. Yes, trimming is tiring and boring, but I try to not do it all at once, no room for mistakes here. Usually I make four units of the same color, dividing the quilt in quadrants, laying out one quadrant, changing the layout until I like the layout. The other three quadrants will mirror the first. This works great, unless I am making a two color quilt. I use a box to keep my units secure and clean.