Everyone has a reliability and performance score, so yes, if you don't know the individual, check them before accepting.
Because the reliability and performance score really shows the correct picture, right? No way it can show how they will behave in exactly your battle, right?
And even if someone has a good performance score: if they only want to fuck up your battle for some reason, their score wont say anything about that. Fight a couple of battles normally, get a normal score. Go to one small battle and fuck it up for them, get a small penalty to score. Fight some more battles normally, your score is back to normal. Guy whose battle you fucked up was still fucked up.
Besides, IIRC performance score was based on how well you do compared with the rest of your team. Since the strategus page only shows KD and not score, I'm assuming it uses your KD.
So if you stand at spawn and just punch/th people there, you will have a 0:0 score. If you take a ladder, put it on a tree and just stand there there the whole time you get 0:0 score. People fighting normally will get a KD ratio below 1 (asssuming even skill and equipment between teams) because they will be damaged before starting to fight because of them having been teamhitted. This will mean the griefer gets better (but realistically, even) score as the people who actually fights.
EDIT: I also forgot to mention how annoying it is to manually check the people you hire. Remember that you will have to go on the normal cRPG website to check someone score and on the strategus website to hire them.
Also add in that many don't even KNOW about the score system.