Curt,
Using your example of under 8.5 -108 compared to under 9 -139
you would have to achieve sufficient ties at 9 to warrant the
extra vig:
So, if 55 games our of 100 are under 8.5, and you were to
achieve 10 ties at under 9, you would now have a record of
55-35-10 at -139 which would be bout the same as 55-45 at -108.
On a line of 8.5, can you expect 9 to hit more than 1 in 10?
If so, go for it!