You can probably dump 6 unless your target audience includes low-budget groups that have no control over their hardware. In the US and Canada this tends to mean indigenous communities, especially schools. If that's not an issue, get your stuff to work in 7 and you'll have done as much as the average person can hope for.
:: shuffling papers ::
I think Iqaluit finally upgraded about a year back, from MSIE 6 to 8-- maybe even 9, hard to tell-- and that was my biggest pool of antiques. Not sure if the pre-existing 7 got upgraded, though. fwiw, I've got some extra jiggery-pokery for MSIE 5 and 6 and haven't seen any humans in recent memory.