you can do both. go back as far as you can go and curl yourself forward. really flex your neck muscles. its easy to cheat and just pull yourself with your legs or push with your arms. so you have to concentrate on basically doing a crunch with your neck muscles. though crunch isnt a good word to use when talking about the neck. you can also just go into the bridge position, take your hands off the ground and count to 20 or 30 or whatever. when going side to side, for sure keep your hands ready to support you.
so, both are good. static holds and moving back and forth, side to side. just make sure to purposefully flex your neck meat. this is also for safety.
I rock my head back and forth and to the sides when I'm doing front bridges, but when I'm doing regular bridges, I try to make it an explosive motion. I start with my back on the floor with my legs bent and try to move my hips up as fast as possible. I sometimes pause and rock my head at the end of the motion, but I don't get to carried away with it. I don't see the use in a static hold, we used to have to do them in high school. I just ended up with a sore head.