Skin test positive (20 mm) after 72 hrs., no afb on sputum, no cough/fever, but occasional blood in sputum (2-3/wk). Doc says not to worry, should i?

Yes. Your TB test is definitely very positive to TB and blood in sputum is always of concern. At the least, i think you need a chest x-ray tofind out what is going on since a negative AFB in sputum does not always rule out active TB and there may be other conditions which can cause the blood in sputum.