Tar isn't even what causes cancer, ostensibly. Furthermore, Cannabis itself dosn't really, the tar comes from combustion (rather incomplete combustion).
Not all radioactive isotopes are created equal, which maybe you know. There is alpha, beta, gamma radiation along with x-rays. Alpha emmiters aren't so toxic unless they are ingested/get inside tissues (e.g Po-210).
The main problem is, with most pot growers it is never a "little" of anything. Just because "pot isn't good for you" we should end it right there and load our plants up with toxins? Alcoholic beverages aren't "good for you"; should distillers neglect to remove the methanol, or start adding a little more?
Like I said, if you grow for yourself then by all means. If you grow for other people, especially paying customers or medical patients then yes, as with any product sold for human consumption there is a DUTY to produce it as clean and toxin free as possible (which means not adding any more toxins). If you don't think so it's on you and karma.
I just don't buy the "so what everything causes cancer", "it's a drop in the bucket", "nothing is good for you anyways", "adding a little strychnine to your nachos wont kill you" sentiments.
On another note, I knew a girl in Elementary/Highschool whose father worked with radioactive isotopes (either medical or technology-oriented). He died young, of cancer.
...And poor Madame Curie.