Eddie,
Rumor has it that chemical companies make most compounds to reagent grade specs, but only bother to test batches that need to actually be of such high purity. It is the testing that adds the additional costs to ensure purity. While they probably don't produce every compound in this manner, and some may require additional or special procedures to reach reagent grade, I don't think you'll see any practical difference in the silver nitrate. The purity of the lower grades is probably more than needed for your purposes. As an aside, the purity of stuff we use today is probably much, much higher than what was available to most photographers 150 years ago when they were figuring all this stuff out.