Don

Of the verses where it can be discerned, the LXX is used more than the Hebrew text, much more.  However, it is not sure why for the gospels in some cases, it might be Jesus or the author choosing to use the LXX.  There is a website that gives the info, seach on LXX new testament.

In Judea and the Galil they used Hebrew text, but in the diaspora they used LXX.  Jesus was clearly familiar with both and seems to have chosen the one that most clearly agreed with his point.  Recall that ANY translation involves interpretation so the LXX includes interpretation in its translation choices.