I would say its a habit... A lot of guitar players have that one move that they do... Stevie Ray Vaughan had the shoulder shrug pucker face while he would do a nasty run... Petrucci always puts his foot up on a footrest when he has to do a hard part of any Dream Theatre song because he's mainly classically trained and he plays best when he's in the classical "sit down" position... Zakk Wylde and Slash (among others)tend to tilt the guitar towards the heaven... Its a stage presence thing... No one likes a guitarist that just stands there.