It's natural to develop stiffness and tension in your joints after long periods of remaining relatively motionless. It should be common sense to everybody that you should always take regular breaks, if only to get up, stretch and walk around. If you decide to try to crack your neck or back, do it properly. If you are hesitant however, simply do some relaxing stretches.
I notice some people, when they play games, also physically move in real life. My wife, for instance, when she plays Minecraft, will move her head around in all kinds of ways, even though it has no effect on the in-game character's ability or movements. When I point it out, she says she can't help it, it's not like it's a volunteer thing for her. I know I don't do the same, because I've "trained" that out of me with hours upon hours of gaming, combined with conscious awareness of that kind of behavior. Now, the reason I mentioned the story about my wife is that maybe you're doing the same thing? It really wears out her neck, and if you're the same, it probably will do the same to yours. Just a possibility I wanted to throw in there.
