Hi Scott
If you have a recipe that says you are boiling for lets say 60 mins and the hop additions are at 60 mins and 30 mins and 10 mins as long as you stop the boil after 60 mins they willbe as per your recipe'
If you boil off for longer than the 60 mins this will obviously evaporate more water and concentrate the wort also the hop additions will then become longer which will alter the strength of the hop flavour and increase the bitterness.
If you do decide to do this you would need to remove the hops after the 60 minute boil to keep the recipe/hops constant as it should be