the tactical knife lowers your accuracy by a little bit, but i'd say its well worth the kills it takes to get it.
I'd say if you want to run around 1 hit killing everyone even thos that just gubbed 1000 painkillers, then get the knife.
If you want your pistol to be a trusty backup weapon for when your ammo runs out, use any other attachment
(im on my 2nd prestige and I always unlock every pistol attachment as soon as i unlock the gun

)