Hmm, common (others doing such), that's hard to say.
You can prioritize traffic (if there's congestion), as it egresses your device going to the ISP. Almost always, Internet devices will not "honor" your ToS markings, so there's often no benefit to such markings. Usually, though, Internet devices will not change your ToS marking, so it might be useful at the other end of your Internet transit.
Also usually, many Internet providers work very hard to insure traffic is not subject to congestion while crossing the Internet. However, that leaves four typical bottlenecks, your link to the Internet, both ingress and egress, and the "far side's" connection to the Internet, again both ingress and egress. What you're doing only addresses one of those usual bottlenecks, so when working with "sensitive" traffic you're still much at risk unless you can totally control/manage (i.e. QoS) ingress and egress traffic for both "ends" of your Internet path. (NB: the latter can be done if you only use your Internet connections for traffic to/from your other Internet connections, alone.)
