Backing up for clarification- If using simply as a mono toggle function while mixing or to ck. mono compatibility, I would have it placed on the master bus.
If I were using other functions, such as the previously mentioned "safe bass" I would likely insert it on a track or sub-mix bus.
For mixing, I often have three basic utilities inserted on my master, adding frequency spectrum analysis and more detailed level metering.MASTER BUS INSERT:MONO BUTTON:
Everyone's needs and preferences vary, This but one recommendation because it is simple and free.
There are many other stereo or Mid/Side processors that could be used to accomplish the mono toggle for example, and there's many options for metering etc. as well. You kind of have to explore a bit and see what suites you personally IMO. For example there are spectrum analyzers built into many parametric equalizers, and many metering options with various features. Those shown are but a few options, yet come well recommended as a starting point.