According to this answer, for the question How often does ISS require re-boosting to higher orbit?,
During Space Shuttle years, small re-boosts were also performed by the Shuttle Orbiters (according to Wikipedia, they had 232 kg of fuel available for that)...
Which engines did it use for this purpose? I think, using the forward or rear reaction control thrusters would cause lateral stresses on the docking port, due to the torque provided, since the thrust vector doesn't pass through the centre of mass of the whole system (ISS+Space Shuttle Orbiter). Or in other words, if these engines are fired they will cause the ISS to spin about its centre of mass, in addition to the re-boost (which will be less effective as at some time the engines will fire retrograde due to the spin).
The following image shows the Space Shuttle Endeavour docked to the International Space Station (ISS), flying at an altitude of approximately 354 km (220 miles), and was taken by Expedition 27 crew member Paolo Nespoli from the Soyuz TMA-20 following its undocking on May 23, 2011.
It can be seen, that neither forward or rear reaction control thrusters can't be used alone. They must be used in addition to other thrusters to nullify the torque, and I think that will be an energy-intensive process. Instead, they could have transferred the fuel from the orbiter to the propulsion module of the station, where the thrust vector is along the centre of mass and doesn't cause any torques. I think, there were no thrusters powerful enough facing the heat shield side to provide orbital re-boost. So, How was the Space Shuttle Orbiter used for the same?