Since my question didn't yet receive an answer, I did some digging around of my own. This should be considered as a temporary answer, before more precise ones will be actually possible.
The answer is, that for the time being, we don't know yet, because we don't yet know what orbit around 67P/Churyumov–Gerasimenko will be first attempted, or is indeed the most desirable one. And we likely won't know until the Rosetta spacecraft is awaken from its hibernation mode it's currently in (entered on 8 June 2011) to conserve energy, and it makes its own observations of the 67P/Churyumov–Gerasimenko.
Approach itself will be based on ground observations alone, due to onboard camera capabilities, but orbital maneuvers after approach will be determined based on Rosetta's observations, too, presumably during last stages of the approach in May 2014:
The deep sleep is made necessary by the craft's enormous distance from
the Sun and the weakness of the sunlight falling on its solar panels,
which cannot produce enough electricity to power the probe fully. (Source: ESA)
It is scheduled to be awoken from this deep sleep, or hibernation, on January 2014, when it will begin its preliminary observations and a controlled slow approach to the comet's core, where it will enter a slow speed (relative to comet's core) orbit around it, to about 25 metres per second. Calculations to Rosetta's approach and orbiting trajectory will be made synchronous to its own observations:
As Rosetta drifts towards the heart of the comet, the mission team
will try to avoid any comet dust and achieve good comet illumination
conditions. The first camera images will dramatically improve
calculations of the comet’s position and orbit, as well as its size,
shape and rotation. (Source: ESA)
Artist’s impression of Rosetta approaching the nucleus of Comet 67P/Churyumov-Gerasimenko. Source and hi-res image at: ESA
Rosetta spacecraft's approach and orbit will be operated from the ground, but the spacecraft does have ability to react to observed conditions on its own, since the communication with the ground control can take up to 50 minutes in a single direction:
Rosetta will be operated from the ground. It is impossible to have the
manoeuvres for the whole mission programmed before the launch because
adjustments will have to be made at each stage of the journey. At the
appropriate time, commands will be sent from Earth to adjust the
spacecraft’s trajectory. But instructions from the ground can take up
to 50 minutes to reach the spacecraft, so to overcome the delay,
Rosetta must have the ‘intelligence’ to look after itself. This is
done by its on-board computers, whose tasks include data management
and attitude and orbit control. If any problems arise during the
cruise, back-up systems will ensure that the spacecraft can remain
operational during critical mission phases. For example, the
spacecraft will automatically position itself with the solar panels
facing the Sun, to avoid it becoming powerless. (Source: ESA)
Short of the last two quotes above, I wasn't able to find any additional source of information on what orbit around 67P/Churyumov–Gerasimenko would be considered most desirable, or estimated trajectory corrections for the spacecraft due areas of increased particle density it might be flying through, especially as the comet approaches the Sun and slingshots past it (November 2014 – December 2015), and the coma and tails increase in size and density.
