As I understand it, in many cases, the surface of the tea is washed before it's processed. So I imagine the risk would mostly be pesticide residue that's absorbed by the tea plant, rather than what's on the surface of the tea. As to whether any of that would be released during the rinse, and thus washed out, I'm not sure if there's any credible research on that -- I wouldn't count on it, though. I'm no chemist, but just as all the caffeine isn't washed out during the rinse, I wouldn't expect that any toxic substances necessarily would be either.
Ultimately, I don't try to worry about it too directly, especially since there's not much (if any) tea I'd want to drink that is certified organically grown by any certifying authority with any teeth at all. I try to buy from credible and trustworthy sellers who work with small producers, and hedge my bets a bit by not buying all my tea from one source.
If you want to reduce your overall pesticide consumption, changing things in your diet is probably more logical than changing things in your tea consumption.
I think as time goes on, there will be an improvement in these certifying standards in the main tea producing countries. But even so, as with farms in the US, certification is often too large a burden for the smallest producers (who, as with farms growing produce), often produce the best product.