OP, .X and .X.X updates are always safe. It's the X.0 updates that you gotta look out for.
THIS THIS THIS. Updating to 9.1, 9.2, 9.2.1 when you are already on iOS 9 for example brings very little risk. Most of the time these updates improve things rather than make them worse.
However updating to 10.0 or something, or looking back, going from 7.1.2 to 8.0 is much much more risky. I have faced performance issues and many bugs every single time I've updated to an X.0 update, 4.0, 7.0, 8.0, and 9.0 in my case.
I started my iOS journey in the iPhone OS 3 days, with an iPod touch 2nd gen. With that I went to iOS 4, hugely regretted it as my device was slowed to a crawl, but I was forced to live with it until the iPod 5 came out. I got the iPod 5 and it came with iOS 6. From there I went to 7.0 and saw issues. Got an iPad mini 2 which had the most current SoC at the time. That even had issues with 7.0, constant springboard crashes and awful UI bugs and slowdowns. By 7.1.2 things were pretty great, but I updated to 8.0 and suffered many bugs and performance issues for months again until about iOS 8.3. Also got an iPhone 6 during that time and it was pretty good with iOS 8 aside from the bugs. Then I updated both my iPhone 6 and iPad mini 2 to iOS 9 and I have had performance issues ever since, 9.3 however seems to be making good improvements, and 9.2 already did so as well. Still more work to be done though.
See the cycle? If you can tolerate waiting on those big X.0 updates, do it. Based on the cycle Apple is following now with many X.X updates, I'd wait till X.2 or X.3 before updating. Updating to X.X or X.X.X updates are fine as long as it's the same "main version" like 5.0 to 5.1 or 6.0 to 6.1 is fine. 5.1 to 6.1 might be a little more risky however.