The 'dry" firing without film is a good way of keeping the shutter in shape. However,with film in the camera there are differences. There is "drag' on advance and sprocket gears which helps ensure that lubricants are dispersed evenly on gears and shafts. I keep some old films around for this when dry-firing. Just run them through until the camera feels smooth. It is also a way of checking that the advance and sproclets are in synch. take an old "dead" film and with a marker draw lines on the film, using the filmgate as a template (remove the lens first!). Now draw the vertical lines for each frame (both the left and the right one). This will give an idea how the spacing is and you can transfer this film to another camera and by loading it carefully, you can use it to check that cameras spacing and advance!
One trick that i use when travelling for extended time is that every week or two weeks, I will go and buy a short (12/24 exp) roll of color negative film. I will load the camera and shoot through all the speeds and aperture at meter indicated exposure. Have it processed and look at the machine prints. They should be close to each other in color density. That means that the exposures are correct across the board. With mechanical cameras you have speed variations, particulary at the higher speeds and occasionally the shutter can start "capping" (not open either partially or remain closed completely). This way you can identify at what speed it does it and avoid them until you get the camera fixed. There is nothing more dis-heartening than coming home from a trip and finding that one of your cameras has decided to act up and cap at 1/250 or 1/500 or simply that the advance is acting up and you have over lapping shots. This way is not fool proof but it does give you a bit of confidence in the camera. It is a cheap way of ensuring that everything is OK.
Another way is that I ALWAYS shoot a couple of rolls with the bodies I am taking on a trip the week before leaving. A quick process and check and I know that they
work.
Service intervals for me is usually a CLA when I buy a used camera (I simply factor that in to whatever I paid for the camera) and then every 5-7 years they are checked out and CLA'd again. I do cycle through my cameras on a regular basis and at the moment two of my M2's are in for service and one Nikon Rf is going in next week. That should take care of this years services, unless i find yet another M2 to add to the stash!