It's very unlikely that the force of outgassing would be sufficient to shift Rosetta or Philae's position on the comet. When Rosetta was initially approaching 67P, the comet was estimated to be losing about a liter of water every second; averaged over a sun-side surface area of roughly 50 square kilometers, this amounts to something like .02 milligrams of water per square meter per second evaporating. The exact force exerted would depend on the velocity of the departing water molecules, but it's obviously not going to amount to much. Further, note that evaporation will be much slower directly beneath the spacecraft, because the surface will be shadowed.
Note that Philae has been at rest on the surface of 67P for about two years, a period which included 67P's closest approach to the sun, and therefore its most furious outgassing. It is not known for certain that it hasn't shifted position significantly, but its position in pictures taken from Rosetta in September of 2016 is consistent with its estimated position in mid-2015.