RE: Tuning ?
Always mark the cam as you have described. And don't let anybody kid you. It's more important to do it with single cams because the longer string can creep more and cause a bigger change than what you would normally see with a dual or Binary system.
I don't quite get your question about tuning. You ask how often to GET the bow tuned? Maybe you're asking about how often to get things checked. The truth is, if you shoot a lot, then you owe it to yourself to learn how to do your own work. You get familiar with this and you can take some measurements, such as brace height, A2A, draw weight, draw length, nocking point hright in relation to the rest, peep height above the nocking point. Then you can periodically check these if the bow changes somewhat. The more you do the more you'll learn how to do and understand what does what.
For what it's worth most of the above is NOT tuning a bow. It's mostly SETTING the bow up. Proper tuning is asjuting all this stuff to get the best arrow flight and performance out of the bow and the only person who can do that right is you. You have to be the shooter. Nobody shoots with the same grip or form so if another TUNES the bow it may not shoot the same for you.