PPD tests are wildly inaccurate. I know for a fact that it only catches 40% of the people with TB, that is, if you have TB only 40% of the time will the PPD test say you have it.
Unfortunately, I don't know the reverse data off the top of my head (ie if the PPD test says you have it, what is the percent of the time that you don't actually have TB). All that a positive PPD test means is that at some point you were exposed to the mycobacterium genus (the group that includes tuberculosis). Thus, you may have active or latent TB, or you may have had a BCG vaccine (common outside the US), or you might have one of the many other mycobacterium members. You just don't know with a PPD test.
About 1/3rd of the world tests positive for latent TB. Thus, it is quite common for someone to be PPD positive. It doesn't mean that you are sick or that you have that much to worry about. But if you are PPD positive AND you are showing the signs of active disease, then you should worry and seek treatment.
I'd love to get ahold of a sputum sample of yours and run it through some of the tests that I'm developing. But I know that the regulations wouldn't allow for that. I can only work with samples that cannot easilly be linked back to specific individuals.
Unfortunately, I know little about the treatment side of TB - I work solely on the diagnostic tests. Isoniazid has about a 0.5% chance of side effects if I remember correctly. So, the chance of developing liver problems is there but small.
But like others have said above, if you start taking a cocktail of drugs, please take them all every day for the entire time you are prescribed. This is to help save the lives of millions of people who will die from MDR-TB or XDR-TB (the multidrug-resistant or extremely drug-resistant forms). Heck, if you don't care about them, then care about the small chance that you might develop one of those resistant forms if you don't take your medications. If you are worried about isoniazid, you'll be damn worried about the second line of drugs. They require injections and hospital stays for months if you can even live that long.