MRSA, like all staph, lives on the skin, in the nose, etc. So when there is a break in the skin, the infection can get out of hand. If you have one problem already in an area like your tooth, your body can be more susceptible (sp?) to opportunistic infection.
I personally don't use neosporin...so if it's something that gets bad enough that tea tree oil or gentian violet can't treat, I would get it drained and cultured.
How large is the area? Does it feel like there is lump underneath? You can draw a sharpee line around the red area and if it starts growing or doesn't respond to treatment, then get it checked out.