You must have an old version (1.01) of the A7V. I believ that ASUS recommends that the thermistor be placed on the back of the heatsink, as close to the center as possible. Another option is to carefully cut the excess plastic from the tip, and mount it against the edge of the actual core. (make sure that it doesn't interfere with the heatsink to core fit). The dge of the core will probably give you slightly more accurate readings but depending on the BIOS version that you use they can be off by 10c or more.
BTW don't use TBIRDS faster than 900 on this version of the mobo, they tend to burn up.