for non QAYG..i finger press seams while constructing a block, then iron the block when complete, check its squared up at this stage..i join my blocks into rows and press the completed row..then join the rows, pressing again, then press the whole top, this time using steam..
for QAYG blocks and quilts, pressing is the key to getting a superb finish, 'press-as-you-go' is a must, don't cut corners and the resulting quilt will shine!! :-)