Depending on what your playing; i.e. your level of comfort with the material, it might make a difference. *Whenever you see a jazz cat or something like that playing and improvising his butt off, he(she) will almost always have it strung relatively high. *I know I do whenever I'm doing challenging stuff like that. *When Im just jamming to a blues tune, or doing rhythm work, the guitar can be much lower. *It's almost as if having it strung up high gives you a more concentrated feel. *I know I will probably get flamed by people saying that Vai, Satch, Gilbert, Petrucci, Johnson, etc. all have their guitars strung up at pretty much medium heights. This is true, and they are no doubt playing difficult things, but they are very comfortable with what they're playing, and many times it seems like their live playing is very similar to their recorded playing. What I mean is, for example, Vai's FTLOG solo sounds VERY similar on several live recordings I have of it as it does on the album. *This is a feat in itself, reproducing such a challening solo, but I believe it all comes very easy to him. *He does not have to really focus all his attention on thinking up what to play, and playing over changing key centers, modes, etc like he would if he were totally improvising. *You can also see this in his level of showmanship and stage presence. *When he's totally making up stuff, he is much more calm and reserved, whereas when he's playing stuff he's done before, he's a maniac!
Whew, that was a lot of rambling. *In short, put that thing wherever it feels comfortable for you. *As a rule of thumb, the more challenging your music is for you, it might help to have the guitar higher, thus closer to your eyes and brain