I used daconil before any problems and still got cankers -BUT I believe that the combo of spraying before and after with it saved the upper 2/3rds of my toms. I started my first spray when the toms were about 8 inches high.

the other things I've heard about avoiding disease from the start is rotating the place where the tomatoes are planted each year and always providing fresh soil. I tried to find sterile potting soil for my containers - not easy, but I knew right from the get go nothing had been added to "enhance" thereby letting me know that nothing had been added to make them sick either.

Sorry to say I know nothing about BT.

Bt is safe at any time. I've also never heard of Daconil burning plants, so I wouldn't worry there either. Just don't mix them -- it might be wise to alternate a couple days apart, so that you don't immediately wash off one treatment with the other. In a pinch I'd do the Bt followed by Daconil, since the Daconil needs the most even coverage to work.

--Alison