Reniceing daemons seems probably always good and never bad.
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Also possibly ionice, although it looks like ioniceness is derived from CPU niceness by default.
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According to the ionice man page:
| For kernels after 2.6.26 with the CFQ I/O scheduler, a process that has not asked for an I/O priority inherits its CPU scheduling class. The I/O priority is derived from the CPU nice level of the process (same as before kernel 2.6.26). |
| The priority within the best-effort class will be dynamically derived from the CPU nice level of the process: io_priority = (cpu_nice + 20) / 5. |
This suggests that you don't need to run ionice explicitly for modern kernels, but you should make sure to nice +5, at least.
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This was a very long time ago, but I think the root cause of whatever was happening was really conflated with some other issue.
Modern daemons have new features like "autoscale" to help manage resource usage, and we haven't run into other issues where using nice would solve anything since this. I'm just going to close this out until we see more use cases; this is probably not worth the complexity if it only comes up half a time every few years.