Originally I was a huge fan of 5/5 str no matter what.
However the more and more I think about it now I tend to lean towards going DEX.
Here's my SA shark bite setup,
As you can see, my +str is actually more than my +dex and that doesn't look good at all. Mithra doesn't have this same problem since i believe you are are sitting at 80ish DEX and you are really lagging in STR. 77 str total definately doesn't cut it
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Here comes my real reason going for 5/5 dex:
Nowadays, if I feel like parsing on my Thf for max output, I pop meat. But the problem that comes with it is accuracy. Mire is fine and I can definately parse 93% with minimal acc, but when it comes to mamools, trolls, colibris I would have a hard time keeping it near 80-85%. Since everyone knows during meeleeing, STR almost does nothing and the inherit +2.5 atk from it is kind of negitable when you are having 600ish attack. ( /nin, red curry + 1 minuet)
On the other hand, 5/5 dex gives you 2.5 ACC + ~.5% crits. In a meat build on tougher camps, you basically have to have around 55-60 acc for it to work , on top of dagger merits.
You may think that 2.5 acc is not that much, but originally for TP I have to use 2X sniper rings to keep my accuracy effective, thus unable to utilize my shinny rajas ring which has incredible stat on it. Now with dex merits + rajas, you can comepletely replace one of the sniper, keep the same acc, AND having the store TP bonus and sublte blow kicks in. Also on top of that you get +1% crits from the combined +10 dex from merits + rajas. IF you are using BD/SK combo you'd understand the pain that I have for sitting in 99% tp 90% of the time and have to wait one more round to reach 100%.....
So now having brutal + rajas just reduces those incidents for like 50%, and now I am a much happier thief. =) I do agree with grays that if you are a Hardcore-Die-Hard-Thief-for-Life thief, I'd go for 5/5 dex. But then again the pre-requisite of it is you have to eat MEAT for its bonus to be worth while. i.e. +15 extra dmg from SA the whole time, making use of that +2.5 extra accuracy.
Otherwise if you are using sushi.... just stick with str merits