There is no option, you must submit your article to PMC if you have received an NIH grant. In terms of copyright complications, we are fortunate that almost every single journal in existence has at a minimum agreed to allow submission of their articles into PMC. However, save yourself much worry and just check to be sure.
The easiest "roundabout" way to find out is to look at your journal's "method" of submission. Alternatively, or more directly, just go to the journals homepage. Most journals display this information somewhere in a "for authors" or analogous section.
Does your journal say nothing? If they do not have a policy in place, they will likely be willing to work with you to find a solution so that you can publish with them and be given permissions for adding the article to PMC.
This website lists many journals and their open access policies, including NIH information: http://www.sherpa.ac.uk/romeo/