Is it important to keep the mouth closed during meditation? I sometimes find this a little uncomfortable. How to you normally position your mouth/tongue while meditating? Is it o.k. to practice with your mouth slightly open?
If anything is uncomfortable for you then it's best not to do it. You don't want to be paying any attention to anything other than the mantra. So where you position your tongue isn't important either. Just let it go where it naturally goes. If you can't breathe through your nose then breathing through your mouth will be ok. If you are having difficulty with a blocked nose would Jala Neti help? http://www.aypsite.org/313.html
On the other hand if you are doing Spinal Breathing you can position your tongue in kechari mudra. http://www.aypsite.org/108.html However I might be a bit early in suggesting that. So it might be something to consider later on.
I really wasn't able to meditate deeply until I learned Kechari Mudra, so I suggest keeping the tongue in one variation of Kechari ( which can be as simple as touching the root of the upper teeth with the tongue) and breathing through the nose. If that is difficult then just do what you are able but keep it in mind as something to work towards.