I have had experience on a couple of mountain bikes when I could not adjust the disk brake caliper positions far enough left or right to keep the pads from rubbing the rotor a little. Is this common and/or normal? Is there a something I'm missing, or should I just bend it a little?
You are not alone. These brakes require constant adjustment and are not fit for purpose. The recommendation is to upgrade to Avid BB7's or a hydraulic system. Read the litany of complaints:
Before you tear them off the bike in disgust you may want to take a read of the Park Tool instructions for adjusting Avid mechanical disk brakes:
This explains quite clearly how the brakes actually work - the outer pad moves whereas the inner pad does not. The disk gets pushed across by the outer pad to hit the inner pad. Note that the more expensive BB7's allow more adjustment and that the even more expensive hydraulic systems work differently, pushing both pads out to meet the disc rather than just 'bending it across'.
Good rim brakes are better than cheap disc brakes and your bike unfortunately has the latter. I recommend that you upgrade the brakes to the best mechanical system you can afford rather than spend any more time trying to adjust what sounds to me to be a flawed product.
Ultimately after 3 months of average riding you will get some rub from disc brakes, because of the heating and cooling the disc gets slightly warped and shaped. If it is slowing down your wheel, as opposed to just heating it, then I suggest you get it serviced (bled and adjusted). I have pad control on my Avid Juicy 7's and only use it little, once dialed in it's where I like it for control purposes.