Accuracy is not really that big a deal for the DIYer, for obvious reasons a commercial service company would want accurate gauges but for us DIYers it does not matter if the gauge is off 10-15 psi. The suggested IP is usually a range to start with and even outside of that range the req will perform fine. It may not be up to full specs in max flow for example but that value is way above what a diver can breath so it really does not matter.
Your assumption that repeatability and the ability to determine if the reg is locking up at a consistent IP and not creeping is spot on, those are what we as DIYers need to focus on rather than worrying about a specific IP value. I personally pick an IP somewhere in the upper area of a regs suggested range and use that for every reg. Picking one gauge to be your "official" gauge and using it consistently is a good idea. What we as DIYers need to know is if the IP reads the same on our gauge as it was the last time we checked it, the fact that it may in reality be off 10 psi due to gauge inaccuracy is not really important. I set all my regs to one IP value for this reason, that way I don't have to remember what reg is set to what. In the field, I am not really worried about the exact IP but rather is it locking up consistently and not creeping. If it looks high or low but otherwise OK, I'll check it against my reference when I get home.
