It depends on how much power you plan to dissipate with the transistor. If you plan to put a lot of current through it, you'll be dissipating more power, which means more energy is wasted in the form of heat. It is this heat for which the heat sink is used, to prevent damage to the transistor. What voltage and current do you plan to switch with the 2N3055? This will help a lot, as we can easily calculate the power and determine whether or not a heat sink is necessary.
Generally the heat sink is used as the collector, as it is already bolted to the chassis of the transistor. However, you could simply put a small bolt and nut through one of the connection holes, wrap a wire around it, and tighten it down. I have also seen some people who soldered a wire to the transistor case. It depends on what will work best for you.
I hope this helps!
Regards,
Der Strom