In what context would I use 见怪不怪?
Oxford uses the word inured, which should give you an idea of the negative connotations attached to it.

not be surprised by anything unusual
对腐败见怪不怪, 腐败就难以根除。
If we become inured to corruption, we won't be able to wipe it out.

Adsotrans defines it as

 to be totally jaded

Which is a good way of understanding the feelings behind 见怪不怪.
Here's an example sentence from a Chinese Chengyu dictionary

他的行为开始令人觉得怪异,但时间一长,人们也就见怪不怪了｡
His behavior made people feel most peculiar, but after a while, everyone stopped seeming to notice.