It 100% doesn't hurt, and can definitely be a help. Authors know their stories better than I will, and can highlight themes that I might miss in the rush. But it doesn't need to be exhaustive, and shouldn't be something that authors burn a bunch of time on. And Jack's points on consistency are very true, I'm sure I apply tags differently than Gromet would have.
The category thing is a bane for me, I hate having to pick one especially when stories are wide-ranging in theme, but for now I'd agree, best to stick with the one category for a series. That can be hard in its own way, for example it's quite common for self-bondage / caught stories to morph into regular bondage stories with no SB aspects in the later chapters. There's probably no perfect solution, but it feels like it could be better.