It all depends what software you are using to place them and why/how you want them.
I.E. are they to be part of a component or do you want them as stitching vias between planes?
It depends on your Thermal requirement . each component has its own thermal dissipation factor please check the data sheet. If you like to see exactly you have to simulate with tool. you have to use thermal via must in some cases. near high power dissipation section on your PCB.