I recently purchased a Q6700 and a tuniq tower 120. I've built systems before but the screwing system for an LGA775 I've only had experience with twice.

When installing that screw set for that, is it better to screw the tuniq tower to the motherboard as tight as possible or make it loose?

Considering there are 4 pins that you screw down, if you screwed too tight would it create a hole or a space where there is no contact between cpu and heatsink in the center (considering the pins force the clips on the side lower than the point of contact for the CPU/heatsink)

I wouldn't make it as "tight as possible", because that would be too tight. However, I also wouldn't make it loose. Just tighten the screws all of the way, but do not overtighten them.

good rule of thumb here is the 2 finger rule. Basically just use your fingertip of your index finger on the scewdriver and turn .. and no wrapping your finger around the driver on the last turn.

If it doesn't seem tight enough, though it should..u can use your middle finger too