Not much difference labor wise. If you get an extension, you can still drill your vertical holes thru the top plates from the ground. But yes, an extra step up and down x1000 or more on a (heavier) ladder on rough in, putting in can lights, etc., does slow things down a little bit. Figure maybe 5-7% more labor; that's a ballpark guesstimate and not from any software or estimator programs.
What will really slow you down tho is vaulted ceilings, stairwells, exterior lights where you need to get wayyy up there. From my experience, homes with 10' ceilings on the first floor tend to have more of these kinds of outlets and fixtures.
I have a few lightweight aluminum ladders I use for higher ceilings. No need to carry around and move a heavier fiberglass one umpteen times if you dont need it.
