Is it necessary to replace your stock shocks if you buy lowering springs for tC? I've read some comments in here that they've blown their stock shocks with like Tein S-Tech. Whats a better drop the Tein S-Tech or Tanabe DF210 for tC?

 

Both questions can be answered with a search. Here are 2quick links for the stock shock/lowering springs question:
https://www.scionlife.com/forums/viewtopic.php?t=94705
https://www.scionlife.com/forums/viewtopic.php?t=93581

For the other just type Tein, S-tech, Tanabe, or DF210 and you should get a ton of posts.

As for some quick answers, here you go.
It is not necessary to replace the stock shock when install lowering springs but it is recommended. The lowering springs will shortern the life of your stock shocks. How much they will shortern it depends on the drop, spring rate, road conditions, etc.
As for which is a better drop, some people prefer the s-techs and some prefer the df210s. Do a search, read some post, and decide what works for you.