Just wondering if there was a particular point at which people decided to have 2 embroyos transferred instead of 1?

My IVF specialist has said 4 or 5 failed attempts is the point in which she suggests that 2 be transferred. Just wondering what other people's experience has been? Can't decide if the idea of twins is lovely or terrifying (especially if they have significant health problems - am afraid that I would feel responsible and awful forever after for choosing to have 2 transferred if something went wrong).

Any experiences/advice/thoughts would be much appreciated!