Heat is definitely beneficial to help in the removal of air and to help the penetration of resin into the break, however it is not necessary.  While you can use a probe for these conditions, we feel heat is safer and easier. 

Heat can be used after the injection and vacuum cycles to help fill any areas where there is still air remaining.  By heating the inside layer of glass it expands and pushes up on the top layer of glass, opens up the repair allowing the resin to flow in. 

On a star break this is done on a pressure cycle which pushes the resin in as the leg opens up.  On a bull’s-eye you start the heat process under vacuum and after heating immediately go to a pressure cycle. 

Important:  If you can’t keep the back of your finger on the glass after heating the glass is too hot.  Always let the glass completely cool before curing.