a SMD-resistor usually distributes its heat to the PCB board.
So I would not worry about. Except when you use elevated temperatures.
In this case you would probably have to do a derating above 70 deg C.
Exactly, the heat is transferred to the PCB, so the resistor doesn't have to dissipate much heat at all. An 0603 will dissipate it's 1/8w when soldered to a board, safely, over quite a wide operating temperature. That said, by virtue of thermodynamics, an 0805 resistor rated at 1/8w will dissipate it's heat over an even larger operating temperature because it is a larger package with a larger surface area and a better thermal junction to the board.
I wouldn't worry about it, even with an elevated temperature; you'll run into issues with other components first most likely.
Additionally, to a certain extent, potting/resin can IMPROVE localized heat dissipation, by offering a lower thermal resistance path, eg a single resistor in the center of an empty 10x10mm pcb will cool quicker in epoxy than in free air, because the epoxy will pull the heat away from the resistor quicker than air. However, if the board is a densely populated SMD board with power devices, the natural convection currents of air would provide vastly better cooling.