Gene editing: how to stay on-target with CRISPR. Vivien Marx. Nat Methods. 2014 Sep 29;11(10):1021-6. doi: 10.1038/nmeth.3108.
In addition, let me reiterate some rules of thumb about minimizing off-target effects:
1. 3 or more mismatches in the 20-base protospacer are highly likely to prevent off-target cleavage.
2. Or, consider using truncated protospacers of 17 or 18 bases. The catch is you will probably have to pick a protospacer with a G as the first base to allow guide RNA transcriptional initiation.
3. I'm not sure if paired nickases are the best way to go yet. A better way to think of it is that it may depend on the specific goal of your CRISPR experiment.