If you don't isolate the thermocouple from the heater block then it is likely to pick up noise. Also, heater cartridges occasionally fail with a short between the element and the heater block, so unless you isolate the thermocouple you risk getting a short between the heater voltage and the thermocouple input.
I recommend that you get the E3D cartridge thermocouple and compatible heater block, then you can be sure of isolation and good thermal contact. You may wish to buy the E3D silicone sleeve at the same time.
