I just got done refolding and fondling a bunch of fabric in my stash. 90% is LQS with some Joanne's and Walmart thrown in for good measure. Some of the fabric is a bit thicker than others and some a bit thinner. As Dunster said, the fold takes up some space -- and the thicker fabric takes up a bit more. What is important is the finished size of your block. You need to make the blocks in a quilt a consistent size so that you can assemble them and not throw the pattern off (like Madquilter said -- you may lose some points). It is best if you do a test to see what you have to do to get a consistently sized block.
To do this, I cut practice pieces from the fabrics that I am going to use -- 2.5" x4.5' works. Stitch them and measure them. The block should measure 4.5"x 4.5"". If it doesn't, then make adjustments in your needle position or in the edge you run your fabric against. I use a lined note card -- the lines are 1/4" apart -- to get a good visual and I mark the information about where the needle should be positioned so if I go back a week later I can remember what I found and don't have to start over again with the measurements.