Assuming a 30-06 firing a 168 TTSX @ 2750fps mv, th difference between 950' ASL and 8950' ASL at a 600 yard target is 1.6 MOA or about 10 inches less drop. Windage will differ, too. Those numbers are assuming identical temperature and other variables. It's ip to you to run your specific numbers and determine if there is enough of an issue for you to worry about. If you're only shooting 300yards it's probably not going to matter.
