If you have a working qos system (such as the ones in openwrt, cerowrt, ipfire, edgerouter, and multiple other fq_codel enabled systems) I'd recomend having it on all the time. I note that the first shipping 1st party fq_codel enabled product is actually streamboost, which is quite good, I'm told, and in multiple high end gaming routers today. In all cases you have to do a measurement and then derive the correct settings.
Peak loads happen all the time. A single web page load can saturate your connection, briefly, and mess up your interactive traffic. A systained load, such as that from a big upload or things from bittorrent, will also mess up your interactive traffic badly. Even a mild amount of jitter induced by other traffic can make a game unplayable.
As for measuring with ping, ping is just a proxy for (dns,voip,new flows, etc) getting a feel for how well interactive traffic will perform under whatever other load you are running with. Holding jitter of ping-like packets below 30ms is needed for good voip and gaming performance. Increased latency leads to talking overrunning each other, with about 250ms total latency being worst case desirable performance for voip in particular (you start stepping on each other's conversation in an otherwise natural pause).
The classic "bufferbloat" test is one flow up, one down, and one ping. We do a variety of more complex and useful test in the rrul test suite which is available as part of "netperf-wrapper" on github.
As for examples of the benefits of using a good aqm/packet scheduling system such as those in the products above: