I shoot the 175s out of my 12 twist .308 with no difficulties (in F-Class TR at 1,000 yards) at all, as do lots of other folks in Houston, Texas (before I moved to Vegas). The rifle and load works fine here at 2,200 ft altitude too.
If memory serves, the US Army is using an 11.25 or 11.5 twist in their M24s. I do not believe a 10 is required, but it will not hurt, especailly if you think you may go to teh 200s or want to shoot the 220s at some time in the future.
Were it my rifle, and I was not going to shoot anything heavier then the 175s, I would go with either a 1:11 or a 1:12.