The important thing is to get the finished unit to be the size you want it to be.
Some of the variables - along with the seam allowance are:
Cut size of the pieces - there can be variations - it is a good idea to use the same ruler or cutting system for the whole project
Was the fabric washed before cutting - some fabrics shrink a bit when they are steam pressed
Fabric thickness
Thread thickness
The moving target explanation is one of the best - and most comprehensive answes - that i've seen.
The three-strip test unit is worth the time if super accuracy is important.
Last edited by bearisgray; 01-13-2014 at 06:11 AM.