There is no defined school of thought on this, as every actor, agent and casting director will have their own preferences.
The main purpose of your headshot is to be an honest representation of you, this can reflect your cast-ability subtly or strongly based on how you want to market yourself.
It is worth thinking hard about your casting and the type of parts you want to play and are suited to before your shoot so you have a clear idea. Speak to colleagues, teachers and friends, and get some honest opinions.
As much as you may think that you can play everything, the industry unfortunately doesn’t work like that. It's a sad truth, that in a highly oversaturated market, people will want to pigeonhole you and put you in a ‘box’ so that they can know where you fit.
This however is something that you can use to your advantage if you can figure out your casting brackets, and it’s also something that agents will love, as it shows that you understand what kind of actor you are.
These casting brackets, if you can work them out before your shoot, will allow you and your photographer to be on the same page, and if needs be use this during your shoot, and get the shots that will work for you. Remember, the ultimate purpose of your headshot is for you to make the short list and be called in for an audition.
Remember, the camera is your link to the casting director. If you are nervous about ‘playing to the camera’, don’t worry, your photographer should work with you to get the best out of you, the best thing you can do is bring a positive and laid back attitude with a heap of energy, and enjoy the process, headshots shouldn’t be a chore.