I worried about this a great deal, and then a friend pointed this out...
Take two common factory loads - say Federal Vital Shok (with Nosler ballitic tips). 150gr in .308; 130gr in .270. Zero both at 185 yards.
Drop at 300 yards: .308 = -8.8 inches; .270 = -7.26 inches (or thereabouts - actual results will vary, but not by a significant amount).
Now bear in mind that the width of your cross hair probably represents in the region of 2-3 inches at 300 yards. So at the *absolute* screaming maximum I would ever consider taking a shot on a live animal, the difference between the two is so marginal that I am effectively incapable of adjusting for it. That is assuming a perfect shot: no wind, totally rested shooter, on a completely solid rest etc etc.
I stopped worrying and instead went looking for the nicest rifle I could find in either calibre. Got a .308 and am delighted with it.