I often run into this problem when working with charity blocks made by many in a group. One of my favorites is to sash just 2 sides of the block.... with about a 2" finished sashing for a 12" block.... alternate columns... column 1 is top and right, then top and left, top and right etc... column 2 is bottom and right, then bottom and left... etc. repeat for each column. (if your blocks have a top and bottom to them, watch out for which way the block faces).
Once your sashings are on, you can re-trim to a consistent size... once the blocks are stitched together, they are offset from the previous row and column, and your size difference is VERY HARD to find. this also adds great movement to your quilt. you can find a picture of a recent top I did this way here:
http://i529.photobucket.com/albums/d...1-20080137.jpg
Another way is to add about a 3" frame around each block, and then turn them wonky and retrim to a consistent size... The quilt pictured here had blocks that varied as much as an entire inch....
http://i529.photobucket.com/albums/d...18161407-1.jpg on this quilt, I had to "chop" some corners...as it was a donation made to our group by an estate, and the original piecer had not been careful with her points originally..... I made 3 quilts out her blocks using this method.... they will all go to charity, and I think they still turned out great!
http://i529.photobucket.com/albums/d...18161534-1.jpg
Good luck with your project...... Hope this was helpful!