You cap Pets Expertise at 26 with 9% Hit rating on your DK (this is something i remember i read long time ago somewhere while learning about Unholy DK).
You can test it yourself and do couple of raids with 8% hit, than with 9% hit, and compare the pets miss/dodge ratio...
Also, 9% Melee Hit = 11% Spell Hit.
11 + 3% from Virulence + 3% from Spriest/Boomie debuff = 17% Spell Hit, aka no miss on DC, IT, Pesti and DnD.
Its completely up to you what you want to go for...
You can get 8% melee hit (263), 9% Melee hit (295) or 11% Spell Hit (289).
End result will be the same, aka, it wont change your DPS almost at all if u decide to go 8% or 9% melee hit...