On stock twins there really is no safe boost. The twins will eventually fail and running a higher then stock boost will only shorten there short life span.
Make sure you watch to see if your car ever starts to smoke at idle once it gets too operating temperature. This is an indication that the oil seal('s) have worn out and are leaking oil into the hot side of the turbo.
I personally am running a 1jz with stock twins and wont be pushing the turbo past there stock boost pressure of about 10-12 psi.