These are two fantastic resources for guide design and CRISPR screens; taking both on-target and off-target activity into account when designing guides!
Optimized sgRNA design to maximize activity and minimize off-target effects of CRISPR-Cas9. Doench et al. Nature Biotechnology 2016. link
Unraveling CRISPR-Cas9 genome engineering parameters via a library-on-library approach. Chari et al. Nature Methods 2015. link