A chain should indeed wear evenly on each link. Only if there is a problem with the chain drive, ie...links with violated rings, damaged sprocket teeth, damaged links due to bad link removals, etc...will a chain wear unevenly.
The reason you will see manuals and tutorials explain to check the measurement in several different places on the chain is so you can identify a problem chain, rather than because they wear unevenly naturally.
I realise that bike mfg's will provide you with a method for checking your chain slack and it usually amounts to making the measurement the easiest for the rider to accomplish. But, the proper measurement for chain slack on ANY bike is to have approx. 2% distance of your sprocket shaft centers when the chain is in it's tightest possible point. To get the chain in it's tightest point you have to either remove the shock and pull the rear up or compress the shock until the chain is at it's tightest point and keep it there with a strap. Once the chain is stationary in it's tightest position,(swingarm in a straight line between counter shaft and rear shaft) you can lay a straight edge accross the top of the chain at both sprockets and measure from that line to the top of the chain at it's lowest slack point.
You should really only have to do this procedure one time. After you have the correct chain slack then put the bike back in it's normal position that the MFG recommends you measure, and check to see what measurement you get. Use that measurment from then on. It may vary slightly from what the MFG recommends, but it will be a much more precise measurement for your particular bike.
I have seen bike mfg's give out the exact same measuring procedures and values when they have changed the swingarm length from year to year. This is proof that they are providing method for ease of rider rather than using engineering data of what the slack should actually be for a particular model. However, if you don't pay close attention to detail, you should just stick with what the mfg spouts and ride.