After collecting data by surveying the area, I edit OSM using JOSM and use Bing as a supplementary source.
I set the source tag with
“Survey 20120906 and Bing 2008”.
i.e. the survey was on the 6th September 2012 and the Bing data is from 2008. 
I think this may be useful because the Bing imagery may over time become outdated and future data collectors will then know where the data came from and update accordingly. 
As this is constant for the session in progress it would be nice 
if the source information could be added by default.
Does JOSM have such a feature? I could not find it but I could have missed it!
If not where can I add a Nice-to-Have feature on a wish list.
First, you should set the source tag on your whole changeset instead on individual objects. Otherwise it is not clear to which tags the source applies if the objects get changed later. It is also a lot less work, of course.
Second, there is a bug tracker for JOSM where you can create tickets and propose new features. However the list of open tickets is rather long and some of them are really old, don’t expect your wishes to get implemented soon unless you submit a patch yourself.
I would not appreciate or use the feature you would like to have since I often not only map the roads I traced, but also fix stuff (e.g. turn restrictions, missing layers of bridges) in the region I work at. It would not be helpful to have any source tags of the survey on such objects.
Since the autocompletion of JOSM works quite well you could try to use it:
press ctrl-a to add a new tag to the object, type ahead “source”,
press tab,
choose by pressing arrow-down or typing ahead.
Or you just paste your previously copied source tag using ctrl-shift-v to the objects.
hth
malenki