Justin,
I have done about 13 sinew bows and agree with Pat B that backstap is easier to process. but I like to use leg sinew, buffalo if available.
I usually use the leg also since its cheaper for me to obtain overall.
After processing leg sinew, I like to soak it in water, squeeze out excess water and lay it out prior to sinewing a bow.
This seems to give a more uniform layer with the coarser leg sinew, especially deer sinew.
Dave