If you are making complex blocks, with more seams in some directions than in others, then you need to pay attention to the size of each patch after it is sewn. Generally this means you must have a scant 1/4" seam. If you're just making 4-patches, or 9-patches for example (same number seams in all directions) then the seam allowance doesn't matter as long as it is consistent.
If you're having trouble with fraying, you may be handling the fabric too much. You might try starching your fabric to cut down on the fraying. I seldom run across a fabric that frays enough to be a problem. Once the top is quilted, the fraying should stop.