for swaps- when a 12 1/2" block is supposed to be swapped there is seldom any (tolerance)---a 12 1/2" block is expected- if you tend to not make your blocks precise you would be best off to make them a little large-then trim to the proper size before sending for the swap- often if your blocks are not correct they are returned unswapped.
if i am working on something for myself i try very hard to make my blocks the size they are supposed to be- even an 1/8" off will add up to a whole inch in only 8 pieces---if my first block does not come out right i figure out why- if i continue to have problems i try to adjust so they are larger & can be trimmed to size.
if your blocks are continuously 1/4" off it only takes 4 to be a whole inch off---if following a pattern that can really cause nothing to work- it is generally best to strive to get them right- not allow yourself a (tolerance)