I have been upgrading my dumb switches with some new matter based smart ones. My home was built in 1997, so there are ground bundles in the boxes, but the existing switches do not have a ground connection.
When upgrading single pole switches I obviously connect ground to the new smart switch.
What is the best approach for grounding when upgrading to a smart 3-way or 4-way switch where the old non grounded dumb switches work in conjunction with the main smart switch?
Ground wires are intended to carry current only when there is a failure in the electrical system that would give the current a path through your body.
Given that ground wires are a protection against failures and not part of the operation of switches or other equipment, you should not have any second thoughts about connecting ground anywhere.
If your switch has a screw to connect a ground wire, use it (unless you know for a fact that the ground will be picked up by the connection to the metal box). If it has a factory-attached ground wire, definitely use it.
This rather depends on where you are, and therefore what kind of switch you have.
Some jurisdictions' switches are completely encased in plastic, with no metal - not even screws - showing on the outside. There is no way for house power to get out of the device if a wire comes loose and touches something it should not be touching.
USA switches (likely others as well) have metal screws holding the switchplate covers on. These screws attach to the metal mounting strap of the switch body. If a loose wire touches that strap, house power can get to the person touching the switchplate screw and give them a dangerous shock.
Effort should be made to ground any device being wired in that has metal on its outside.
Additionally, if the manufacturer put a screw or wire for ground on the device, then there is a foreseen possibility that there is a risk of electric leakage through the device. They don't put in extra parts that drive up manufacturing costs that aren't needed.
TL;DR: If there is a ground wire or ground screw on the device, ground it. There is a reason the ground point is there.
If you're planning to open up and ground the "dumb" switches I recommend replacing them. For the amount of effort needed to rearrange the wires, you may as well spend a dollar or two for a brand new switch at the same time.
Also consider in cold or dry climates with carpeting, adding the ground may change your experience with the switches. Carpeting can cause static buildup that will discharge if your fingers are near a grounded surface such as a faceplate screw. One good way to prevent that is to get the "screwless" type faceplate that has no exposed metal parts.
