And of course I always stand up when I play a live gig or something, and I never get problems from standing.
One thing you're probably missing, as other folks said is the strap adjustment..
I used to play the guitar real low (like james hetfield) although I always played lead... but I usualy got problems with tapping and very wide finger streches (used to get over that by hanging my guitar like Slash during solos)... on the other hand , playing the guitar that low made me more comfortable with my picking hand !
Now I wear my guitar like Steve Vai or a little lower and I feel very comfortable playing any technique (tapping/streches/sweeping/fast picking/etc.)... but if I wear it as high as John Petrucci or Jason Becker I feel more strain in my picking hand (not very hard, but it's a little less comfortable)...
You should take care what makes you less comfortable while standing , is it the picking hand or the fretting hand ...
And you should always keep in mind that strap adjustment for being comfortable and playing good is much more important than just lookin cool.
