Diagnosed with cellulitis on left shin. Today is 7th day of 10 day Bactrim. Decreased Redness and swelling (2-3" in diameter day 1) do I need 10 days?

Yes, you need to. follow the health care professional's advice to the letter. Stopping early can lead to a rebound that might be much more serious or extensive. You wouldn't want to eventually lose your foot just to save three days of antibiotic therapy!