with block swaps you will get some good ones and some not so good ones. i make all of my blocks with perfection in mind (and something i want to receive)that i will swap and that includes using quality fabrics, trying to my best to make sure points/seams match, that the block is the correct size and that it is aesthetically pleasing.
while we may work to achieve perfection, sometimes points are off or seams get a bit misaligned. i think this just adds to the character of the block. if it is too off aesthetically, i will take the block apart and resew it.
some of the block swaps include quilters of all levels so keep that in mind also. i say make your best blocks the first time around. if i am trying new techniques, then i use practice fabric first to make the block and correct any errors before i will make it up in the fabric for the swap.
there are so many talented quilters on this board, when you're in a block swap you will receive some beautiful works of art. give it a try.