I'd say that for beginners the time lost looking at the timer is almost negligible, perhaps like 1-2 seconds at the most? And if F2L is already completed, then you dont really need to keep constant eye-contact with the cube for lookahead, especially if you have already recognised and in the process of executing your OLL alg. But yeah, as you get faster, every little pause will add up and in the end, its best to get rid of them as soon as you can.