I don't worm mine with safeguard monthly either, but I do know a lot of people that do. I think a lot depends on what the dogs are doing as well. Like if they are in daycare a lot surrounded by other dogs all the time or if they're always out on trails and are bad at sneaking bites of things.
And a lot of the commercial heartworm pills include an antiparasitic, so they are being wormed monthly that way.
Also, I might add: I leave 2 weeks between ivermec and safe guard. So if I am doing both that month it's ivermec on the 1st and safeguard on the 15th.