One of the schematics that I often find when working on my NES is this one.
A few years ago, when I was messing with the CIC chip, I found a few mistakes with the drawing of the reset circuit. I didn't think much of it at the time, so I threw it away after I was done.
Fast forward to this year: I was trying to diagnose why my NESRGB board wasn't working. I dug up the same old schematic and found even more mistakes this time. Usually the designator or the pin numbers were just wrong. There were also no designators for transistors or caps.
Then when I was trying to create a development cartridge, I was confused about a couple of pins. Different sources claimed the A13 pin did different things. The schematic helped a little (even though the pins 37 and 65 were wrong) and was eventually able to figure it out.
I'm currently working on a development cartridge, so I'm going to start with the NROM and UNROM cartridges. I plan to move to working on the motherboard eventually.
I'm going to start by referring to different schematics first; comparing them, looking at datasheets, and seeing if it makes sense. If anything doesn't make sense, I'll trace it out with my multimeter on a NES. Then, once I get the basics, I will verify the schematic vs PCB using my multimeter again. I don't currently plan to use any photo editing software or any pictures of the PCB. Although I may, if the need arises.