Three things to check:
1) Cutting to the correct size (use the same ruler, or same brand of ruler for your entire project if possible. use the same part of the line on your ruler when you cut. and make sure you are actually cutting against the ruler).
2) Seam Allowance. We talk about a 1/4" seam, but you don't measure the seam, you measure the resultant unit. Here's how to check:
http://www.quiltingboard.com/tutoria...ce-t89997.html
3) And finally pressing. Make sure that you are not pressing a tiny fold at the seam.
It's always a good idea to check as you go along, measure the units as you construct them, don't wait until the block is all done. A lot of this is just gaining the skills. The more you quilt the better you are going to get.
I wouldn't worry too much about tossing the blocks you started with (well actually I wouldn't toss them, they would go in my scrap bag). It sounds like you are beyond that project, and if you suspect the pieces weren't cut correctly they are never going to go together. Why aggravate yourself?