When you are approaching the aggro ceiling as a DPS class with no threat dump, you want to maximize the amount of damage per threat (DPT).
subtacting weapon damage from each,
HS8 = 138/(145 + 138) = .4876
HS9 = 157/(157 + 173) = .4757
HS10 = 176/(176 + 193) = .4769
So, we can see that ranks 8-10 of HS have similar values for DPT; similar enough that the variance in weapon damage with each swing will probably overcome the small difference. Therefore, the highest rank of HS is still your highest amount of damage per attack that has the lowest amount of damage per threat.